* Forums are disowned on user account deletion (their owner_id=0)
* A forum without an owner shows a notice at the bottom with a link to petition
to adopt the forum. It goes to the Contact form with a special subject.
* Note: there is no easy way to re-assign ownership yet other than a direct
database query.
* Code cleanup
* Alphabetize the DB.AutoMigrate tables.
* Delete more things on user deletion: forum_memberships, admin_group_users
* Vacuum worker to clean up orphaned polls after the threads are removed
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