AvatarServices now require an AvatarSettingsManager class when

being instantiated. Previously, this was an optional argument that

defaulted to the default AvatarSettingsManager which throws

NotImplementedError for all its methods. However, registries were not

guaranteed to return any third party avatar services instantiated with

the correct settings manager.
This has been corrected by making the avatar services registry always

store instances instead. It has simplified the API as now most things

can be done via method calls with the AvatarService class as an

argument instead of its avatar_service_id attribute. It also has the

benefit of making unit tests more succinct.
The AvatarServicesRegistry's get_avatar_service, for_user, and

configurable_services nethods have been re-written to return an

instance of the requested avatar service, instantiated with the

correct settings manager class.
Unit tests & the rest of the avatar services suite have been

re-written in light of these changes.

Ran unit tests.

Tested avatar services still work with Review Board.