Define and use a route for the /menu-position/{rule} path

[star-szr]

I created a menu position rule and then moved it around in the menu UI (changed its parent). I then got into this unrecoverable exception:

Symfony\Component\Routing\Exception\RouteNotFoundException: Route "" does not exist. in Drupal\Core\Routing\RouteProvider->getRouteByName() (line 187 of core/lib/Drupal/Core/Routing/RouteProvider.php).

I think if we define and use a dummy route this may help. We probably need it anyway even if it doesn't fix this particular bug.

[midnightLuke]

Just flagging this as related to #15