Use mediawiki.ui.button instead of mediaiki.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.
* Echo does not use any mw.ui features other than buttons.

Change-Id: I429d8f2424d6218439eed01fe5ea6497e7697ba2