• Guest users should not be allowed to post to streams marked “Only
  organization full members can post.” This flaw has existed since
  the feature was added in Zulip Server 3.0.
• Permit outgoing mail from postfix; this resolves a bug introduced in
  Zulip Server 3.2 which prevented Zulip from sending outgoing mail if
  the local mail server (used mostly for incoming mail) was also used
  for outgoing email (MAIL_HOST='localhost').
• Ensure that the upgrade-postgres tool upgrades the cluster’s data
  to the specific PostgreSQL version requested; this resolves a bug
  where, now that PostgreSQL 13 has been released, upgrade-postgres
  would attempt to upgrade to that version and not PostgreSQL 12.
• Replace the impenetrably-named ./manage.py knight with
  ./manage.py change_user_role, and extend it to support
  “Organization owner” roles.
• Handle realm emojis that have been manually deleted more gracefully.