improve CSS for site navigation bar (bug 1082161, bug 1083613, bug 1083614)

* remove unused rules, optimise selectors, and make things more OOCSS-y
* improve navigation hover states (bug 1082161)
* use CSS instead of JS to toggle category drop-down on desktop (bug 1083613)
* add borders to desktop navigation items (bug 1083614)
* fixed CSS to actually hide the link outlines on focus
* remove `cur-cat` class and use more descriptive classes
* remove specificity all over the space to reduce complex cascading
* considerable cleanup to `sprite-background-position` for category icons
Merge pull request #727 from cvan/nav-tweax

improve CSS for site navigation bar (bug 1082161, bug 1083613, bug 1083614)