Let users set __NOINDEX__ & __INDEX__ status

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

Bug: 57167
Change-Id: I30d483b5b6c3df7ee56a52c744bbdc610a01873d
Merge "Let users set __NOINDEX__ & __INDEX__ status"