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

Fixes #12669: Properly reset line-height on date inputs for all sizes; Fix IE8+'s misaslignment of text within date inputs

FIXES #12992

Moving the embed to it's own CSS file, moving the docs to the Components page with an example, fixing a Glyhpicons list problem in the docs

Fixes #12683: Remove the overflow: hidden; from the .panel-group > .panel because it apparently serves no purpose and cuts off nested dropdown menus.