Changed the dropdown navigation to be navigable using the keyboard (if
Javascript is enabled).

- The tab key navigates all menus and submenus in order
- Links in the main navigation bar can be navigated with the left and right
  arrows
- Submenus can be navigated with the up and down arrows and can be closed with
  the Esc key. Pressing left or right opens the previous or next menu

Note: Dropdown navigation works as before with JS disabled and only requires
it for keyboard accessibility.

Change-Id: Ia7aeb33526c71a2fe3ea4c635f4a8aa17c64e21b
Signed-off-by: Jono Mingard <jonom@catalyst.net.nz>