Add users privileges request to API 2.0 to simplify access audit for Admins

We need to know, for audit purposes who all has
access to which of our repositories, and at what permission level.
Currently, this involves separate API calls for getting a list of groups
that have access to a repo. And a call to pull the same for non-grouped
users.

Ideally, I'd like to see a call that implements the access query that
enumerates both groups and individuals, then includes the group expansion
once, maybe in a separate section of the response (a separate call would be
OK for that, assuming I could submit multiple groups).