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; see
  I5bd23434961497d49293956c92b46ab3a6f14a41 .
* ContentTranslation does not use any core mw.ui features other than
  buttons.  If it does in the future, 'mediawiki.ui' can be added.
* Some 'mediawiki.ui.button' uses must work without JS, so they have
  to use addModuleStyles. Thus, the best practice is for all to do so
  when possible to avoid double-loading.

Change-Id: I2e1f894517d5e558099590dd72907b958dcd83ad