(bug 43270) Add wgPostEdit to show if user just saved this page

* Sets short-lived cookie in EditPage
* Adds JS module, 'mediawiki.action.view.postEdit', to all views to
  convert cookie to wgPostEdit.  This avoids caching the variable in the
  HTML.
* Based in part on PostEdit extension

Change-Id: Ie89507f8e98c365df9d93a7633223103b9025790
Merge "(bug 43270) Add wgPostEdit to show if user just saved this page"