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.
