`ReviewableQueuedPost` got refactored a while back to use the more
appropriate `target_created_by` for the user of the post being queued
instead of `created_by`. The change was not extended to the `DELETE
/review/:id` endpoint leading to error responses for a user attempting
to deleting their own queued post.
This fix extends the `Reviewable` lookup implementation in
`ReviewablesController#destroy` and Guardian implementation to account
for this change.
87ebbec9b2