Use mediawiki.ui.button when mw-ui-button is used

* 9e4e58bce8d10f65cf298b39f48e86c41b6a82b8 broke this into a separate
  module (for modularity, and potentially to allow wiki content to use
  mw.ui buttons by default in the future).
* Currently, the button code is in both mediawiki.ui and
  mediawiki.ui.button for backwards compatibility.  However, it will be
  removed from mediawiki.ui when existing uses transition over.
* SpecialMWOAuth.php is using mw.ui buttons and other mw.ui features
* SpecialMWOAuthManageMyGrants.php is only using buttons.

Change-Id: I52ccf49464c714ab5fb8944fb46b5b8fbdc5fc55
Merge "Use mediawiki.ui.button when mw-ui-button is used"