thumb.php and img_auth.php cleanups

* Specifically check isDeleted() instead of relying on the file
  being moved to the deleted zone alone.
* Made img_auth.php skip some unnecessary checks for public wikis.
  Also made it set the private cache control headers only if needed.

Change-Id: If82589acd53c02943d5f3bbafd29381fd63f8f68
Merge "thumb.php and img_auth.php cleanups"