Add a rubocop rule to warn developers to put new routes under -

The following discussion from gitlab-ce!28830 should be addressed:

  • @dosuken123 started a discussion:

    Should we have a rubocop rule to warn developers to put new routes under -? It seems we're still putting new ones under the project route, occasionally.

TODO:

Edited by Dmytro Zaporozhets (DZ)