Expose canned responses for code-manager

[EnTeQuAk]

As part of https://github.com/orgs/mozilla/projects/79 we want reviewers to select canned responses.

For that we need to expose them through an API.

[eviljeff]

idea: replace the current database-backed canned responses models with constants defined directly in the codemanager repo. Editing them using django admin is a bit of a pain, and is so rarely done anyway (afaik) that writing a PR to update them is a similar amount of effort.
@wagnerand and @kewisch to contradict me here.

[kumar303]

There are some additional user stories here which will probably affect how canned comments are stored: #6603