FEATURE: Allow site settings to be edited throughout admin UI (#26154)

This commit makes it so the site settings filter controls and
the list of settings input editors themselves can be used elsewhere
in the admin UI outside of /admin/site_settings

This allows us to provide more targeted groups of settings in different
UI areas where it makes sense to provide them, such as on plugin pages.
You could open a single page for a plugin where you can see information
about that plugin, change settings, and configure it with custom UIs
in the one place.

In future we will do this in "config areas" for other parts of the
admin UI.

spec/system/page_objects/pages/admin_settings.rb

@@ -45,8 +45,26 @@ module PageObjects
         self
       end
 
+      def clear_search
+        find("#setting-filter").click
+        self
+      end
+
+      def toggle_only_show_overridden
+        find("#setting-filter-toggle-overridden").click
+        self
+      end
+
       def has_search_result?(setting)
-        page.has_selector?("div[data-setting='#{setting}']")
+        has_css?("div[data-setting='#{setting}']")
+      end
+
+      def has_n_results?(count)
+        has_css?(".admin-detail .row.setting", count: count)
+      end
+
+      def has_greater_than_n_results?(count)
+        assert_selector(".admin-detail .row.setting", minimum: count)
       end
     end
   end