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.