RCFilters: Improve circle mixin and change to new standard icon size

RCFilters: Improve circle mixin and change to new standard icon size

Improve circle mixin, variables and apply standard icon size:
- Apply new icon size of 20px,
- Divert `.mw-rcfilters-mixin-circle()` into two mixins, applying former
only on circle classes to not repeat output of shared properties for
each color,
- Add new parameter to apply `px` based `min-width`/`min-height` to
circumvent rendering issues,
- Rename variables for unification with WikimediaUI Base vars and easier
readability

Bug: T190980
Change-Id: I76fec57ff237ba02afefcea28916d8348bac9b0e