The User Selector widget wasn't updated for the new avatar rendering

support recently added to Djblets. This change updates the widget to

supply initial rendered avatars, and for the user API queries to request

avatars as well.
The one annoying bit is that form field widgets have no access to the

current HTTP request, so Djblets's avatars support had to be updated to

make this optional. This got us most of the way, but we access the

request in one of the avatar templates to determine if we can provide

the user's full name in the tag. Our template tag for this has been

updated to make the request optional in this case.

Tested with the User Selector with users having different avatars.

Verified that the right avatars were all shown for any initially-loaded

users in the list, for any auto-completed entries when looking up users,

and for any users added into the list.