The original logic is buggy now that emails can belong to (and be
invited to) multiple realms.
The new logic in the `invites` queue worker also avoids the bug where
when the PreregistrationUser was gone by the time the queue worker got
to the invite (e.g., because it'd been revoked), we threw an exception.
[greg: fix upgrade-compatibility logic; add test; explain
revoked-invite race above]
This fixes a regression in 25c669df52.
We were draining the queue in both the superclass and the subclass,
so by the time the subclass started processing events, there were
no events to process. Now the subclass properly uses the events
passed in from the superclass.
The example_user() function is specifically designed for
AARON, hamlet, cordelia, and friends, and it allows a concise
way of using their built-in user profiles. Eventually, the
widespread use of example_user() should help us with refactorings
such as moving the tests users out of the "zulip.com" realm
and deprecating get_user_profile_by_email.