The Django ORM query would not work correctly for pathological
DirectMessageGroups with no Subscription rows. When the Subquery gave
empty results, the UPDATE would set group_size to null - when the point
of the migration was exactly to make sure this column is always set and
allow making the column non-nullable in 0575.
Such DirectMessageGroups can occur as a result doing .delete() on all
UserProfiles that were in the group - or by doing realm deletion via
either .delete() or `manage.py delete_realm`.
The natural choice for group_size of these objects is 0. The simple SQL
query naturally achieves this result, without needing any special
handling for this case.
This commit adds a new `group_size` field to the `DirectMessageGroup`
model, and backfills its value to each of the existing direct message
groups.
Fixes part of #25713
