mediawiki.ui: Bring checkbox and radio on par with WikimediaUI design

mediawiki.ui: Bring checkbox and radio on par with WikimediaUI design

Bringing checkbox and radio on par with WikimediaUI design templates,
also slightly enhancing accessibility by using colors of new palette.
Width & height is taken from current OOUI size, based on 14px Vector base
font-size.
Adding several similar variables to variables.less, which will be replaced
by WikimediaUI Base vars in future.
Also:
- removing necessity for images for radio checked states
- replacing `margin` with `padding` between widget and label in order to
  have `cursor: pointer` on full element

Bug: T148265
Change-Id: I68848d48ea45c67124ac75be58748b4e9fb3085d