As documented in https://github.com/zulip/zulip/issues/441, Guardian
has quite poor performance, and in fact almost 50% of the time spent
running the Zulip backend test suite on my laptop was inside Guardian.
As part of this migration, we also clean up the old API_SUPER_USERS
variable used to mark EMAIL_GATEWAY_BOT as an API super user; now that
permission is managed entirely via the database.
When rebasing past this commit, developers will need to do a
`manage.py migrate` in order to apply the migration changes before the
server will run again.
We can't yet remove Guardian from INSTALLED_APPS, requirements.txt,
etc. in this release, because otherwise the reverse migration won't
work.
Fixes#441.
