Build: Add CSSMin task

- Minify all CSS on build
- Move all minification to a "minify" task
Build: refactor targets

Separate out chained task to more atomically related actions
Merge pull request #2974 from nschonni/v4.0-add-cssmin

V4.0 add cssmin
Jaws polyfill: Added polyfill to fix the Jaws sectioning elements/heading level incrementation bug and the sectioning element/heading association bug.