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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