Closes #13103 by merging it.

Fixes #13066: Add z-index to the form control feedback icons so input groups don't render on top of them

Fixes #13098
Closes #13099

Closes #12850

Closes #12377

closes #12337

Fixes #13003: Move padding on radio and checkbox options from the surrounding div to the label to remove the no-click deadzone caused by negative margin

Move all deprecated vendor prefix mixins to less/mixins/vendor-prefixes.less
Create mixins directory and move partials to that directory.

FIXES #12994