WikimediaUI theme: Set `outline` just once
WikimediaUI theme: Set `outline` just once

We need the same `:focus` state for `outline` once. Removing superfluous
overwrite and move the property on general ButtonElement selector
for all buttons.

Change-Id: I95b8729c1611a979c9e28c0ea1d25691cfb3aea3

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui