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