Update mediawiki.ui button styles

Based on:
* https://www.mediawiki.org/wiki/File:Agora_specs_Buttons_annotated.png
* https://www.mediawiki.org/wiki/File:Guide_Color.png
* http://33cc77.com/wikipedia/ by Daniel Zarick

This has quiet buttons that all start gray, and only reveal
constructive/progressive/destructive color on hover and active.

Also:
* Update KSS docs to reflect disabled state.
* Remove unnecessary text-decoration: none rules (the one in
  a.mw-ui-button should be enough).

Bug: 58297
Bug: 54360
Change-Id: I1dd9bfac65092ad80bb8cd410031c22da0a18305
Merge "Update mediawiki.ui button styles"