mediawiki.ui: Fix quiet button styling and remove mixins

mediawiki.ui: Fix quiet button styling and remove mixins

Quiet progressive and destructive buttons are applied right visual
properties.
Mixins make only sense when they are called several times, it
diminishes readability here and unnecessarily complicates the lookup
of values being set correctly. Therefore removing them.
Also introducing selected variables following WikimediaUI Base naming
convention in order to be replaced easily in future.

Bug: T210115
Change-Id: Iee1559ed185a12bf77fc68b6ef7f10e9a0536b07