add support for generating release notes from upstream release

[shiftkey]

This PR adds support to the script/generate-release-notes tool to read the changelog.json entries where a version represents the first release based on the given upstream release, and generates the release body matching the template that we use currently, with a couple of tweaks:

• Fixes and Improvements now get their own sections
• SHA256 checksums section format changed (to support organizing by architecture)

[shiftkey]

Here's an example of the generated draft release body, using a previous release:

## Fixed

- Entering in double forward slash does not trim the target directory in the cloning dialog - https://github.com/desktop/desktop/issues/15842. Thanks @IgnazioGul!
- In the "No Repositories" screen, controls at the bottom stay inside window when it is resized - https://github.com/desktop/desktop/issues/16502. Thanks @samuelko123!
- Link to editor settings on the tutorial screen - https://github.com/desktop/desktop/issues/16636. Thanks @IgnazioGul!
- Fix crash using Edit -> Copy menu when no text is selected in the diff - https://github.com/desktop/desktop/issues/16876

## Improved

- Improve screen reader support of the "Create Alias" dialog - https://github.com/desktop/desktop/issues/16802
- Screen readers announce the number of results in filtered lists (like repositories, branches or pull requests) - https://github.com/desktop/desktop/issues/16779
- Screen readers announce expanded/collapsed state of dropdowns - https://github.com/desktop/desktop/issues/16781
- The context menu for a branches list items can be invoked by keyboard shortcuts - https://github.com/desktop/desktop/issues/16760
- The context menu for a repository list item can be invoked by keyboard shortcuts - https://github.com/desktop/desktop/issues/16758
- Make floating elements more responsive as the window or the UI are resized - https://github.com/desktop/desktop/issues/16717
- Adds committing avatar popover to see git configuration and ability to open git configuration settings - https://github.com/desktop/desktop/issues/16640
- Password inputs have a visibility toggle. - https://github.com/desktop/desktop/issues/16714
- Welcome flow screen change in context is announced - https://github.com/desktop/desktop/issues/16698
- Focus the sign in with browser button on opening the enterprise server login screen - https://github.com/desktop/desktop/issues/16706
- Show the remote branch name if it does not match the local branch name - https://github.com/desktop/desktop/issues/13591. Thanks @samuelko123!
- Reduce retries of avatars that fail to load - https://github.com/desktop/desktop/issues/16592
  
## SHA-256 checksums

## x64

**GitHubDesktop-linux-amd64-3.2.5-linux1.deb**
15cbb28356054f00edcf06f885e32777194faae9502f9f7520ad1cea62a3755c

**GitHubDesktop-linux-x86_64-3.2.5-linux1.AppImage**
2e453c9142f880f66435917c6ae5aeee825d2ee3bf993b7950ceeeb04a28d43c

**GitHubDesktop-linux-x86_64-3.2.5-linux1.rpm**
fe98f1ed3a46ca160df0b56fb3f49dd20745368a59f9dff22daf703f022361b6

[theofficialgman]

@shiftkey maybe it would be a good idea to make the release notes asset filenames links to the assets. since we can control the order the release notes are written in but the assets always appear in file alphabetical order

basically this

x64
6031515cb44652e573cd231aa2ae151768947f5612cbb98322c3dd2b9ed861d1 [GitHubDesktop-linux-amd64-3.2.7-test7.deb](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-amd64-3.2.7-test7.deb)
cdcb3e2299571488a33e87428ec7d800590fcb411a7577e687976bebbda1fdfc [GitHubDesktop-linux-x86_64-3.2.7-test7.AppImage](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-x86_64-3.2.7-test7.AppImage)
d15c1ab26d9be4725f646f39cd05513ae83643a9f8eca6d70d721f4ec4a1e828 [GitHubDesktop-linux-x86_64-3.2.7-test7.rpm](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-x86_64-3.2.7-test7.rpm)

ARM64
f1f3e4ca6a6c431dd0f80eaa10e35c5b1127847857d517abd605c69798f35198 [GitHubDesktop-linux-aarch64-3.2.7-test7.rpm](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-aarch64-3.2.7-test7.rpm)
ba96bfc5087af6500352b4069091e86fd683e9e74ef1cb8ff15e32ea483f5711 [GitHubDesktop-linux-arm64-3.2.7-test7.AppImage](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-arm64-3.2.7-test7.AppImage)
87b0f9276f903d49bdbbd4b410d797f0c40124cd23c2300a213960741857b750 [GitHubDesktop-linux-arm64-3.2.7-test7.deb](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-arm64-3.2.7-test7.deb)

ARM
34aba140ece5fe97ffe505d3f30c8989a3b3fc665969016df8bf93c56855bb2a [GitHubDesktop-linux-armhf-3.2.7-test7.deb](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-armhf-3.2.7-test7.deb)
428ca152623c3ad73f3738380d188d7118a9406e15c09b8d0a035a64a0abb054 [GitHubDesktop-linux-armv7l-3.2.7-test7.AppImage](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-armv7l-3.2.7-test7.AppImage)
ff0a2be3415bef33ed6e06f8356376478766115196b67745deeda2c692ea2785 [GitHubDesktop-linux-armv7l-3.2.7-test7.rpm](https://github.com/shiftkey/desktop/releases/download/release-3.2.7-test7/GitHubDesktop-linux-armv7l-3.2.7-test7.rpm)

x64
6031515cb44652e573cd231aa2ae151768947f5612cbb98322c3dd2b9ed861d1 GitHubDesktop-linux-amd64-3.2.7-test7.deb
cdcb3e2299571488a33e87428ec7d800590fcb411a7577e687976bebbda1fdfc GitHubDesktop-linux-x86_64-3.2.7-test7.AppImage
d15c1ab26d9be4725f646f39cd05513ae83643a9f8eca6d70d721f4ec4a1e828 GitHubDesktop-linux-x86_64-3.2.7-test7.rpm

ARM64
f1f3e4ca6a6c431dd0f80eaa10e35c5b1127847857d517abd605c69798f35198 GitHubDesktop-linux-aarch64-3.2.7-test7.rpm
ba96bfc5087af6500352b4069091e86fd683e9e74ef1cb8ff15e32ea483f5711 GitHubDesktop-linux-arm64-3.2.7-test7.AppImage
87b0f9276f903d49bdbbd4b410d797f0c40124cd23c2300a213960741857b750 GitHubDesktop-linux-arm64-3.2.7-test7.deb

ARM
34aba140ece5fe97ffe505d3f30c8989a3b3fc665969016df8bf93c56855bb2a GitHubDesktop-linux-armhf-3.2.7-test7.deb
428ca152623c3ad73f3738380d188d7118a9406e15c09b8d0a035a64a0abb054 GitHubDesktop-linux-armv7l-3.2.7-test7.AppImage
ff0a2be3415bef33ed6e06f8356376478766115196b67745deeda2c692ea2785 GitHubDesktop-linux-armv7l-3.2.7-test7.rpm

[theofficialgman]

also since your script is also alphabetizing each individual release, maybe it would also be better to make the order standard. ie: deb then AppImage then rpm for each architecture.

[shiftkey]

@theofficialgman adding this to #901 ? I'm open to adapting the format to whatever makes sense here, just trying to juggle feedback from different angles...