Improvements to {{REVISIONUSER}} handling

Improvements to {{REVISIONUSER}} handling

* Do not change the result to a null editing user anymore.
* Use a new vary-user flag instead of vary-revision. This
  will only cause a reparse on null edits. Normal edits
  can still use the prepared output now.
* Edit stashing now applies for pages with this magic word.
* Fixed bug where the second prepareContentForEdit() call
  (due to vary-X flags) would still check the edit stash.

Bug: T135261
Bug: T136678
Change-Id: Id1733443ac3bf053ca61e5ae25db3fbf4499e9f9