The avatar services code has been cleaned up. Docstrings have been

touched up so that they now document all parameters and do not document

unnecessary ones. The RB.ReviewReplyEditorView now renders the avatar

with a srcset= attribute instead of the data-at2x attribute, which

is no longer used. Finally, a new template tag, {% avatar_urls %}, has

been introduced to serialize all avatar URLs for a given user and

requsested size to a Javascript object. This prevents KeyErrors from

being raised when avatar services only provide 1x resolution avatars,

as well as provides all additional resolutions that they can provide.

(Previously, we were hardcoding 1x and 2x resolutions.)

Able to use an avatar service (the file upload avatar service, in this

  case) that only provides 1x resolution avatars.
Ran unit tests.