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

* 9e4e58bce8d10f65cf298b39f48e86c41b6a82b8 broke 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.
* In this case, both are necessary since there is only one
  addModuleStyles, and both button and non-button mw.ui features are
  used.

Change-Id: I809eb7b0d4b8e49e088bab96d80cf8d72d9d4f90
Merge "Use mediawiki.ui.button when mw-ui-button is used"