* Reorganize forms.less
* Change from attribute selectors to `.form-control`. Few lines, less
specific (meaning easier overrides as `element[type=""]` is more
specific than a class), less intrusive, and more performant.
* Add `.form-group` for basic spacing in vertical forms.
* Remove (unnecessary?) `margin: 0;` from `form` element (as far as I
can tell no browser sets that anyway).