This adds missing type hints to djblets.features, and in the process

updates some of the code to add missing docstrings and to fix types in

some docstrings.
FeatureLevel is now an enum, which should be backwards-compatible

with the original int in how this is used. That gives better type

checking than using an int.
FeatureRegistry has been updated to be a typed registry.

Unit tests pass.
Tested with some code defining a new feature.