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.
* UploadWizard doesn't use any mw.ui feature besides buttons, so
  it can just use mediawiki.ui.button.

Change-Id: Ie5c6bed27869946eda69a8731df3c2fea89ec5b6
Localisation updates from https://translatewiki.net.

Change-Id: I382ee070e5df1aab30dbfe192dde6c283276efd4
Merge "Use mediawiki.ui.button instead of mediawiki.ui"