Use mediawiki.ui.button instead of mediawiki.ui

* 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.
* Only the button features of mw.ui are used here.
* Add comment to explain why addModuleStyles is not used.
Change-Id: I047e3bf6f7f2121f6be87a041c776e6771a697f0
Merge "Use mediawiki.ui.button instead of mediawiki.ui"