auth: refactor user lookup in AuthUser constructor for clarity

First, note that `fill_data` checks that the specified `db.User` is
`active` before copying anything, and returns False if not.

Now, previously when calling e.g. `AuthUser(user_id=anonymous_user_id)`,
`_propagate_data` would explicitly refuse to look up the anonymous
user, but then fall back to the anonymous user anyway (if `active`),
or use None values (if not `active`).

Given the same situation, the new code simply looks up the anonymous
user like it would any other user, and copies data using `fill_data`.
If the anonymous user is not `active`, we fall back to the existing
code path and behave as before (that is, use None values).