Avatar rendering and URL fetching currently takes a request, but this

prevents avatars from being used if rendering outside HTTP requests or

in places where the request is not available (such as in form widgets).
This change makes these arguments optional. Avatar implementations must

take care to respect this, checking if a request is None before

accessing it, and falling back in some manner if needed.

Unit tests pass.
Tested manually (with some changes in Review Board) and verified that

different avatars rendered without having a request present.