This commit continues work laid out by ffec8163b0 for the admin config page for the /about page. The last commit set up the user interface, and this one sets up all the wiring needed to make the input fields and save buttons actually work.
Internal topic: t/128544.
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
