auth: construct AuthUser from either user_id or db.User object

If the caller already has the database User object, there's no reason
for AuthUser to look it up again.

The `api_key` lookup functionality is dropped, because 1) it's only
used in one place, and 2) it's simple enough for the caller to do the
lookup itself.

The `user_id` lookup functionality is kept, because 1) it's frequently
used, and 2) far from a simple `User.get(id)` lookup, it has a complex
interaction with UserModel. (That cleanup will have to wait for another
day.)

All calls of the form `AuthUser(user_id=x.user_id)` can be replaced with
`AuthUser(dbuser=x)`, assuming `x` is a db.User. However, verifying that
assumption requires a manual audit of every call site, since `x` might
also be another `AuthUser` object, for instance. Therefore, only the
most obvious call sites have been fixed here.