Rails has an odd behavior for calling .delete_all on a has_many relation - the
default behavior is to nullify the foreign key fields instead of actually
'DELETE'ing the records.
Additionally, publishing a shared draft topic creates a PostRevision that the
NotifyPostRevision job picks up which is then promptly deleted.
Use destroy_all when cleaning up the revisions and have the NotifyPostRevision
job tolerate deleted PostRevision records.
This takes a small performance hit (several SQL DELETEs instead of just one)
but shouldn't be too much of an issue (high cardinalities range from 30-100).
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