This commit removes the `/admin-revamp` routes which were introduced as a part of an experiment to revamp the admin pages. We still want to improve the admin/staff experience, but we're going to do them within the existing `/admin` routes instead of introducing a completely new route.
Our initial efforts to improve the Discourse admin experience is this commit which introduces the foundation for a new subroute `/admin/config` which will house various new pages for configuring Discourse. The first new page (or "config area") will be `/admin/config/about` that will house all the settings and controls for configuring the `/about` page of Discourse.
Internal topic: t/128544
