Add migration to add existing group creators as owners

[seanh]

Depends on #9032

[seanh]

Not all existing groups will have creators. There are three cases:

1. The user account of the group's creator has been deleted. In this case "group".creator_id is NULL. There's nothing to do in this case.
2. The creator is no longer a member of the group. In this case "group".creator_id will still correspond to an existing "user".id but there will be no user_group row with a matching group_id and user_id. Again there's nothing to do in this case.
3. The creator is still a member of the group. In this case we find the right user_group row and update roles to ["owner"].

And of course we need to not update user_group roles for non-creator memberships.

I'm hoping I can get away with updating all the rows at once as in the SQL query above. In the production DB there are 191,571 rows matching:

SELECT count(*)
FROM "group", "user", user_group
WHERE
  "group".id = user_group.group_id
  AND "user".id = user_group.user_id
  AND "group".creator_id = "user".id;