Add 'mediawiki.ui.button' CSS module

This splits the button styling out from mediawiki.ui into a
separate module.

This is an alternative to https://gerrit.wikimedia.org/r/#/c/87296/

This migration will be implemented in 3 steps, of which this is the
first:
1. Add new mediawiki.ui.button module to core
2. Add mediawiki.ui.button dependancy to all extensions and core
   features that currently use mediawiki.ui (and have buttons)
3. Remove button styles from mediawiki.ui

Change-Id: Ia375d848df15afe6ddc3f3f19e43249d6c0706f0
Merge "Add 'mediawiki.ui.button' CSS module"