Avatar as self-contained app

All code related to `avatar` functionality is optional as per setup
https://github.com/incuna/django-user-management/blob/master/setup.py#L12-16

But then it's being imported into global namespace
https://github.com/incuna/django-user-management/blob/master/user_management/api/serializers.py#L4-6

which makes dependent project to have `imagekit` among its dependencies, which it shouldn't.
This PR fixes that issue by moving avatar related code to `api/avatar`.