This commit converts the Backups page in the admin interface
to follow our new admin interface guidelines.
As part of this work, I've also made `AdminPageHeader` and `AdminPageSubheader`
components that can be reused on any admin page for consistency, that handle
the title and action buttons and also breadcrumbs.
Also renamed `AdminPluginFilteredSiteSettings` to `AdminFilteredSiteSettings` since
it can be used generally to show a subset of filtered site settings, not only
settings for a plugin. Not sure if it's ideal to have to define a new route for this
for every config area, but not sure how else to do it right now.
This commit introduces the initial UI for the admin config area for the /about page. The UI isn't wired and doesn't do anything yet, but we're going to iterate on it in future commits.
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
