Implement file browsing

[EnTeQuAk]

This may need to be splitted into more issues.

File-browsing consists of two parts:

• A file-tree on the (currently) left side that shows all files
• A file-content part on the (currently) right side that shows the content of the currently selected file

API: https://addons-server.readthedocs.io/en/latest/topics/api/reviewers.html#browse

This requires a version_id that should be applied through the URL the user clicks on the reviewer tools page.

[kumar303]

Thanks @EnTeQuAk 🎉 ; I broke it into separate issues:

• View a list of files in an add-on #107
• View the contents of a text file #108
• View an image file #109