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

* 9e4e58bce8d10f65cf298b39f48e86c41b6a82b8 separated mediawiki.ui.button
  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.
* The only mw.ui feature used is buttons, so this uses
  mediawiki.ui.button.

Change-Id: I719714bae9b2fab1f4df355eb92132a24eeceee4