As of this diff all of the platform-specific functionality has been moved out of `BaseEditUserAvatarProvider`, which allows us to consolidate the three previous providers into one.
This concludes most of the refactoring work aimed at decoupling `avatar-hooks` and `*AvatarProvider`s.
Next diff will involve renaming `BaseEditUserAvatarProvider` to `EditUserAvatarProvider` (was initially supposed to be part of this diff, but all the import changes and whatnot made it messy)
---
Depends on D8346