Remove post-edit state detection (in core now)

This change transfers the responsibility for detecting post-edit state and
transmitting it to client-side code from the extension to MediaWiki core, which
(thanks to Matt Flaschen's work in 375687eddb) now implements all the requisite
behavior.

It might make sense to merge the remainder of the extension into core, too, but
I think it makes sense to have the intermediate step (represented by this
patch) of leaving state handling to core but still doing the UI bits in the
extension.

Change-Id: Ifc61b4d3b4581b861cba443adc9d493515a2e395
Merge "Remove post-edit state detection (in core now)"