Allow auth backends to act on a user-list query before it's executed.

Added an overrideable function to AuthBackend which is called at the beginning
of UserResource.get_query_set(), which in turn is called by the GET /users/
WebAPI call.  The default function does nothing.  This allows backends to add
their own logic to fetch users from an external source and add them to the
local user database (or update existing users) before the db query is executed.

I added docs and fixed the doc for another AuthBackend overrideable function
while I was there.

Testing Done:
All tests pass aside from a few VCS-dependent ones, which are skipped.

Reviewed at https://reviews.reviewboard.org/r/5521/