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.