Add convenience methods to the avatar registry

Review Request #8203 — Created May 31, 2016 and submitted — Latest diff uploaded

Information

Djblets
release-0.10.x

Reviewers

The avatar services registry now provides a convenience method to lookup
an avatar service directly by its avatar service ID (as that is all that
is used), as well as a method to get either the requested service or the
default.

In addition, the get_defaults() method has been refactored to reference
a list of avatar service classes on the registry itself to simplify
subclass implementation.

The enabled_services setter now also computes the sets of avatars
services to disable and enable and disables (or enables) each one
individually.

Ran unit tests.