This fixes most cases where we were assigning a user to
the var email and then calling get_user_profile_by_email with
that var.
(This was fixed mostly with a script.)
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.