refactor spinners SCSS

Created by: midzer

Hi all,

latest SCSS code from https://github.com/twbs/bootstrap/pull/22960 did not work for me on spinner -sm classes.

So I went along and refactored a little bit:

• introduce base class %spinner which spinner-border and spinner-grow both extend
• introduce spinner-sm modifier class which is used in combination with spinner-border and spinner-grow to make small spinners, this is the way bootstrap treats other classes as well
• rename both @keyframes to save more byte

Any feedback is heavily welcome. Have a nice weekend (or start in the week) midzer