Ensure that there are no collisions between charts at /grapher/ and Mdims at /grapher/

[danyx23]

Under the /grapher/ path on our website so far we had charts (what we now often call "standalone" charts) and redirects to charts. With MDims we now have another "thing" that lives under this url-space. Because of this /grapher/poverty could either be an mdim or a standalone chart. We should either decide on a naming scheme to discern these or add a safety mechanism (maybe trigger based in the DB?) to make sure that we don't have different entities with the same slug.

[rakyi]

Closing in favor of #4458