Bug 1852020: Cache nav menu for mobile different to desktop

As the menus are slightly different - the mobile one doesn't have
import/export menu items

behat not needed

Change-Id: Ia84f68cbe12a5a6555b7e2e378b44b41875b9328
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>
(cherry picked from commit a1ad5685)