Migrate Extension:PostEdit interface to core

As requested by the VisualEditor team, this change migrates the post-edit
confirmation notice from Extension:PostEdit to core.

This entails porting the 'postedit-confirmation' en/qqq message from the
extension's i18n file (the message key remains the same) to the appropriate
message files and augmenting mediawiki.action.view.postEdit.js to not only
signal post-edit state but also act on it by displaying the confirmation.

Bug: 48276
Change-Id: I01cfc0630c3a505af82922844b5e70c1d61f3c1d
Merge "Migrate Extension:PostEdit interface to core"