Feature/243 calculate sponsored percentage

[NicolasDontigny]

Implemented the calculations for the sponsored progress of a site.

I had to update models, serializers, and so I squashed the migrations to one initial script again. Sponsor progress is now calculated dynamically according to the number of species of a site and of its batches, it is not a field that should be stored in the model/DB

[Samuel-Therrien-Beslogic]

Let's merge https://github.com/BesLogic/releaf-canopeum/pull/257 first for testing purposes

Also remember to link your PR and issue together. You can either use the "Development" menu on the left, or use a "closing keyword" in your description https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword (I created a new PR template to help us remember: https://github.com/BesLogic/releaf-canopeum/pull/258)

[NicolasDontigny]

Your typing error should be fixed simply by adding the return type annotation on your model methods.

It would be nicer if the default dev DB actually came with various progressions (it can be randomized just like before, but on DB creation instead of every get)

Added type annotations