Note that a pretty common use case for this is a realm admin sending this to
everyone after an import from HipChat or Slack. So this adds the realm_name
to the title (so that there is something they might recognize) and kept the
wording generic enough to accommodate the user not having clicked anything
to get this email.
Also strengthens the tests a bit to better test the complicated template
logic.
This was basically rewritten by tabbott, because the code is a lot
cleaner after just rewriting the ZulipPasswordResetForm code to no
longer copy the model of the original Django version.
Fixes#4733.