Let users set __NEWEDITSECTION__ & __NONEWEDITSECTION__ status

Add a trinary option to the page settings pane of the meta dialog that
lets users set the page to have __NEWEDITSECTION__, __NONEWEDITSECTION__,
or neither (and so have default behaviour).

Bug: 57168
Change-Id: I3198409c6aaa4cbce7e26fd56d390ffc91e91eef
Merge "Let users set __NEWEDITSECTION__ & __NONEWEDITSECTION__ status"