FIX: Prevent resurrection of approved queued reviewables when reusing flagged reviewables for the same target

Contribute Bug
1

For these steps:

  1. Set a watched word.
  2. A user triggered watched word, and the topic fall into the reviewables.
  3. A staff approved the topic.
  4. The topic is flagged by a user.

Then the original reviewables of “Approving this topic” will change its status from “Approved” back to “Pending”, and there will be two reviewables in the queue (one for approving this topic, which approve/reject won’t make any difference, and another for dealing with the flag), which shouldn’t be that case.

This commit add logic to prevent that from being changed back, keeping only the flagged reviewables, and some specs are also added to it.