Flow moderation actions in logs

Added a hook to hide Flow deletion log actions in the rare case that Flow has
taken over a non-existing talk page, deletion occured in the Flow board, and the
board is later removed again. The hook is a new commit to core: https://gerrit.wikimedia.org/r/#/c/92504/

Meanwhile also:
* Centralized actions config into FlowActions.php (from PostActionPermissions &
  PostActionMenu)
* Slightly changed closure code for PostActionMenu (change prompted by afore-
  mentioned change)
* Fixed a moderation bug due to '' (moderation status) and 'moderate'
  (moderation action)
* Removed validateRestoredPost (can just use validateModeratePost instead)

Change-Id: Ifb5a36d6915454b17ef42085fd7a3d4c37bbf36c