remove .border-radius mixin and replace with standard border-radius property, since -webkit-* and -moz-* are only needed for older versions

remove modal-open class
hopefully resolve z-index headaches

fixes #5040 manually: remove overflow: auto; from .modal to resolve sporadic modal right padding issues with scrollbars

clean up modals.less comments, add new class for optional use on modal forms to remove bottom margin

change up the z-indexing and add some popover/twipsy content to the test modal on the js docs to help avoid future bugs

another quick pass at ranking all zindexes and listing them in variables for quick overview and editing later

start breaking down patterns.less into more distinct files, update docs for forms to use correct classes