Nested API endpoints improvements

Right now there is at least changes API which is duplicated in multiple endpoints - global and per project, component and translation

The global one uses own viewset with defined filtering and so on, while the scoped ones implement this manually.

There must be a way to reuse the existing viewset in these scopes. This will bring filtering into the UI and avoid issues like #4095 in future (what will become more important as features of the API grow).

This in general applies to other endpoints as well - there are many methods which in fact duplicate viewset functionality. Quickly looking at least following should be affected: ComponentViewSet.translations, ComponentViewSet.screenshots, TranslationViewSet.units, UserViewSet.notifications, ProjectViewSet.components

https://github.com/alanjds/drf-nested-routers might be way to go