improve CSS for category drop-down on desktop (bug 1083625)
Merge pull request #728 from cvan/cat-nav-tweax

improve CSS for category drop-down on desktop (bug 1083625)
fix navbar for android stock browser (bug 1081085)
make position of category icons pixel perfect upon hover (bug 1083625)
use `overflow: scroll` for mobile-width nav (bug 1059002)