[Feature Request] Add Types to our code

[Feverfew]

Is your feature request related to a problem? Please describe.
Dynamic languages are cool and all, but make it hard to figure out what possible types there could be statically. This causes countless bugs, especially as our gencache DB is nullable everywhere!

Describe the solution you'd like
Type check all the code, with CI to prevent type-less code!

Describe alternatives you've considered (optional)
Do nothing

Additional context(optional)
This is quite a daunting project, and there are doubts this will be feasible with Django but we need to experiment before we write it off. We should aim to type check the common and uclapi folders first, as a lot of other apps depend on them. After that we can choose whatever is easiest, just so that we get used to it. In addiiton we can use MonkeyType to do a lot of the work for us! We should probably get CI involved quite soon, so it will check all folders that we know are typed, to make sure we don't regress!

https://mypy.readthedocs.io/en/stable/
https://github.com/Instagram/MonkeyType
https://github.com/typeddjango
https://sobolevn.me/2019/08/typechecking-django-and-drf
https://instagram-engineering.com/let-your-code-type-hint-itself-introducing-open-source-monkeytype-a855c7284881
https://instagram-engineering.com/static-analysis-at-scale-an-instagram-story-8f498ab71a0c