Added ApiStashEdit module for pre-emptive edit parsing

* This lets edits be prepared while users enter edit summaries.
* The edit form will now make use of this API, controlled by
  $wgAjaxEditStash.

Change-Id: I4f4057bc0d1d4a66a8f7cfb7cdc26d443a8eb0c4
(cherry picked from commit 3a6c9d36c9ebf7dea98ebc3f17a44569f2f220de)