R1:550028a8825a

Allow Phriction document edits to be saved as drafts

Summary:
Depends on D19661. Ref T13077. See PHI840.

When a user edits a page normally, add a "Save as Draft" button. Much of this change is around making that button render and behave properly: it needs to be an `<input type="submit" ...>` so browsers submit it and we can figure out which button the user clicked.

Then there are a few minor rules:

- If you're editing a page which is already a draft, we only give you "Save as Draft". This makes edits…