Reorganize mixins and G R U N T

BS3 commits: b69e4d636a43439ae53e2b7782f163b1d1887cf4
Fixes 12669: Properly reset line-height on date inputs for all sizes; Fix IE8+'s misaslignment of text within date inputs

BS3 commits: 2f955907f95ad9b7f42f624eb11142339709c188