Configure dependency injection for API and the database. Uses Dagger (dagger.dev)

The API is held in a holder, so that it can be invalidated or recreated as needed.

Big refactor of the feeds, this introduces a bit more duplication of code (some very similar methods that differ only in the types or minor details), but now the inheritance makes more sense and there is less "forcing" to make the inheritance work (for example: no more casting Int to String just to have the right type for the key)
It should be easier to work with the feeds in the future, maybe we'll find better abstractions later on.