Perform edit stashing when the edit preview or diff form is requested

Perform edit stashing when the edit preview or diff form is requested

* Users are likely to save after they inspect and review their changes.
  The buttons to do so are also located below the edit summary box.
* This obsoletes the backend stashing on preview. Most of those parses
  would have been useless due to being per-section only. Also, some
  extensions like the Graph extension disable stashing for "preview"
  output anyway. Simplify the code by removing that method.

Bug: T136678
Change-Id: Ied77bdbd191dd9267d4295b0fa7b942f65b062db