Review Board now uses avatar services everywhere that gravatars were

previously used.
The admin panel has been updated to reflect this chnage. There is no

longer a setting for gravatars, specfically. Instead, there is now an

entire page of avatar service configuration.

Ran unit tests.
Performed the following manual testing:

- Verified enabling/disabling avatars entirely worked correctly.

- Verified the avatar services admin form saves correctly.

- Viewd the search page and datagrids to verify that avatars render

  correctly.