Add BlockType enum to BlockVersion model

[KevinMind]

Relates to: mozilla/addons#15014

Description

TBD

Context

Testing

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[diox]

You might be able to replace that with just... nothing, and defining objects below with as_manager(), like this: objects = BlockVersionQuerySet.as_manager().

For more complex cases where you do want the functionalty ManagerBase brings we can also define an empty manager inheriting from ManagerBase and use from_queryset() like this: BlockVersionManager.from_queryset(BlockVersionQuerySet)()

If we have custom methods in the manager that somehow don't belong in the queryset but do call queryset stuff, then it's no longer appropriate, but most of the time that should be enough. We have a bunch of old code using the method you used, but for new cases without anything fancy we should try the modern Django way and avoid repetition.

[KevinMind]

This belongs in the docs.