As part of adding support for more than 2B UserMessage rows in a Zulip
server, we need to change UserMessage.id (a field we don't access but
is needed by Django) from an int to a bigint. This commit is a series
of migrations which create a `bigint_id` column and populates it correctly.
This migration will take a long time to run; on chat.zulip.org (a
server with a lot of history), it took about 4 hours to complete.
How to migrate with minimal downtime:
1. Run `upgrade-zulip-from-git` through this commit. It will install
migration 0238 and then more or less hang while applying migration
0239. Once migration 0238 is completed, however, your server should
be able to be started back up safely while migration 0239 is running.
2. Run `/home/zulip/deployments/next/scripts/restart-server` in a
separate terminal to get Zulip running again.
3. When the `upgrade-zulip-from-git` command finishes, it will
automatically re-restart the Zulip server, leaving you in a consistent
state and ready to do part 2 of the migration.
A useful `manage.py shell` query for checking the state after this
commit is consistent is this:
assert UserMessage.objects.exclude(bigint_id=F("id")).count() == 0
Part of #13040.
Mateusz Mandera