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).
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
This feature can be enabled by choosing a destination for the
`shared drafts category` site setting.
* Staff members can create shared drafts, choosing a destination
category for the topic when it is published.
* Shared Drafts can be viewed in their category, or above the
topic list for the destination category where it will end up.
* When the shared draft is ready, it can be published to the
appropriate category by clicking a button on the topic view.
* When published, Drafts change their timestamps to the current
time, and any edits to the original post are removed.
