Add a hook to register avatar services

Review Request #8531 — Created Nov. 7, 2016 and submitted — Latest diff uploaded

Information

Review Board
release-3.0.x

Reviewers

The AvatarServiceHook is a new hook for adding avatar service classes
to the avatar services registry. It is a thin wrapper around Djblets'
RegistryHook, which is a generic hook base class for adding items to
registries from extensions.

Ran unit tests.