refactor: Simplify CSS classes computation
This MR simplifies the way CSS classes are applied to the spinner component by removing the special cases when default size and/or default color are used.
It follows up on @dpisek's suggestion here: !654 (comment 192574849)
Once !647 (merged) is merged, we'll simplify the specs as well.
/cc @dpisek
Edited by David Pisek