- While deleting a user, some of the user's associated records are moved to the
ghost user so they aren't deleted. The user is blocked before these records
are moved, to prevent the user from creating new records while the migration
is happening, and so preventing a data race.
- Previously, if the migration failed, the user would _remain_ blocked, which is
not the expected behavior. On the other hand, we can't just stick the block +
migration into a transaction, because we want the block to be committed before
the migration starts (for the data race reason mentioned above).
- One solution (implemented in this commit) is to block the user in a parent
transaction, migrate the associated records in a nested sub-transaction, and
then unblock the user in the parent transaction if the sub-transaction fails.
1. Have `MigrateToGhostUser` be a service rather than a mixed-in module, to keep
things explicit. Specs testing the behavior of this class are moved into a
separate service spec file.
2. Add a `user.reported_abuse_reports` association to make the
`migrate_abuse_reports` method more consistent with the other `migrate_`
methods.
