LMS: Close Modal Control Uses HTML <button> Element (resolves LMS-582)

This work addresses the bug/issue noted in https://edx-wiki.atlassian.net/browse/LMS-582. The solution is not completely clean as it involves:

* a really bad global styling of all button elements
* overriding the bad styling with an anti-mixin/extend
* a fair amount of !important statements scoped to this UI element (to ensure focus is visually present when needed)