Backend Localization

[MelissaAutumn]

Localization! I hear it's neat. Here's the initial pass at adding fluent-based localization to backend. Feel free to be opinionated on this stuff. If you think strings need to be pulled into separate files based on category let me know! Also let me know if I missed any strings.

Breaking Changes:

• You now have to install the backend in edit mode for tests to run. Simply run cd backend && pip install -e .
• Tests are now ran from /backend instead of /backend/test

Additions:

• Added starlette_context to allow per-request global data
• Added the l10n helper that accepts a msg-id and optionally a dictionary of data for that message.
• Added support for English and German in the backend (Note: We'll switch to en-US in the future.)
• Added a bunch of fluent strings to the file under main.ftl and email.ftl
• Added a variety of API exceptions to exceptions/validation.py

Notes:

• Read up on fluent's file format: https://projectfluent.org/
• There's a lot of duplicate strings for things like model not found. This is based on the advice of https://github.com/projectfluent/fluent/wiki/Good-Practices-for-Developers#prefer-wet-over-dry I'm sure I'm taking that a little too literally so we can work on that in the future.
• I think any API exception that appears more than once can have a exception class. We can fine-tune this and split things out into other files as needed though.
• I went with TB's commenting style for variables.
• Currently the health check with English and German strings are used for a test to ensure l10n helper function is working. We might want to create a specific test route and test strings for this though.
• I intentionally didn't use _ as the localization helper to avoid association with gettext (as the call is a bit different.)
• In the future we might want to store user's preferred locale in the db. But when we roll fluent out on the frontend we can just override accept-languages for now. (At least until we get a worker/queue system for emails.)

[devmount]

Haha it's alright 👍🏻 I'll look over German translation when this PR is out of draft state

[MelissaAutumn]

And it should be good! Feel free to wait until the new year 😅.

[devmount]

Looking good. I added small bug fixes and the German translation. Although _() would have been shorter an cleaner, I quickly got used to l10n(). Thank you, Mel!