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:

• Projects rule !20460 (merged)
• Group rule !21114 (merged)