Refactor the submenu system

Our submenu stuff started getting dirty and separated into different `styl` files, so I've taken the time to do some cleanup.  This will:

1.  Automatically inject close buttons into submenus (using JS)
2.  Cut down on wasted space inside submenus, and makes the "Web Platform" submenu slimmer
3.  Ensures arrows and menus display properly on tablet/mobile and RTL