The user API provides the URLs for any avatar images available, but this

isn't very helpful for avatar backends that don't make use of images

(such as the new fallback backend in Djblets). For those, avatar URLs

will be empty, and the client won't have anything to render.
With this change, clients of the API now have the ability to request

that avatars be rendered to HTML for a given size or sizes. This is

optional, and off by default, in order to avoid sending unwanted data to

clients that don't need it, and because we don't pick any default size

for the avatars that are being rendered. It's entirely up to the

clients.
The implementation supports requests during any GET request (using

?render-avatars-at=<size>,[...] or when creating a new user through

a POST request (using render_avatars_at=<size>,[...]). It handles

invalid values in the list gracefully.

Unit tests passed.
Manually tested with users with different backends, and with invalid

values.