Remove NamespacesController

* removes unnecessary NamespacesController. The main purpose of this controller was redirect to group or user page when URL like https://gitlab.com/gitlab-org was used. Now this functionality is handled by constrainers (like this https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/routes/user.rb#L17-21) and take user to correct controller right from the start.
* serve non existing API routes like `/api/v3/whatever` with Grape instead of Rails. Before this change wrong API url was served by rails with not obvious 404, 405 & 500 errors


See merge request !6733