mediawiki.UI: Align buttons to design guidelines and OOjs UI

mediawiki.UI: Align buttons to design guidelines and OOjs UI

Aligning buttons to design guidelines in https://phabricator.wikimedia.org/M101
and to OOjs UI, similar to https://phabricator.wikimedia.org/rGOJUed9fef533bea
Additionally:
- aligning CSS code to MediaWiki Coding Guidelines,
- adding `transition`s equivalent to OOjs UI,
- replacing deprecated `@colorWhite` variable,
- replacing counter-intuitive `.button-colors()` Less mixin guard, given
our updated widget guidelines has only two primary button types
(`progressive` & `destructive`) left, with separate mixins and
- removing deprecated `constructive` CSS standalone selector and
corresponding KSS styleguide comments.

Bug: T146823
Change-Id: I6b9faefabc62689eabf6f4bccaa3d3756b5b662a