Add the avatar services registry

Review Request #7809 — Created Dec. 15, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.6.x

Reviewers

The avatar services registry is a new object for managing avatar
services, which are defined to provide Review Board multiple ways of
displaying user avatars. Currently, the only supported avatar service
is the Gravatar (http://gravatar.com) service, which is what Review
Board supported previously.

This patch, however, does not migrate to use the new avatar services.

Generated serach indexes have also been added to .gitignore.

Ran unit tests.
Used this in an upcoming patch.