UX: Apply admin interface guidelines to Backups page (#28051)

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.

spec/system/page_objects/admin_backups.rb

@@ -0,0 +1,69 @@
+# frozen_string_literal: true
+
+module PageObjects
+  module Pages
+    class AdminBackups < PageObjects::Pages::Base
+      def visit_page
+        page.visit "/admin/backups"
+        self
+      end
+
+      def click_tab(tab_name)
+        case tab_name
+        when "settings"
+          find(".admin-backups-tabs__settings").click
+        when "files"
+          find(".admin-backups-tabs__files").click
+        when "logs"
+          find(".admin-backups-tabs__logs").click
+        end
+      end
+
+      def has_backup_listed?(filename)
+        page.has_css?(backup_row_selector(filename))
+      end
+
+      def has_no_backup_listed?(filename)
+        page.has_no_css?(backup_row_selector(filename))
+      end
+
+      def open_upload_backup_modal
+        find(".admin-backups__start").click
+      end
+
+      def download_backup(filename)
+        find_backup_row(filename).find(row_button_selector("download")).click
+      end
+
+      def expand_backup_row_menu(filename)
+        find_backup_row(filename).find(".backup-item-menu-trigger").click
+      end
+
+      def delete_backup(filename)
+        expand_backup_row_menu(filename)
+        find(".backup-item-menu-content").find(row_button_selector("delete")).click
+      end
+
+      def restore_backup(filename)
+        expand_backup_row_menu(filename)
+        find(".backup-item-menu-content").find(row_button_selector("restore")).click
+      end
+
+      def find_backup_row(filename)
+        find(backup_row_selector(filename))
+      end
+
+      def backup_row_selector(filename)
+        ".admin-backups-list .backup-item-row[data-backup-filename='#{filename}']"
+      end
+
+      def row_button_selector(button_name)
+        ".backup-item-row__#{button_name}"
+      end
+
+      def toggle_read_only
+        find(".admin-backups__toggle-read-only").click
+      end
+    end
+  end
+end