FIX: Show admin plugin route sub-links in sidebar (#24982)

This changes the Plugins link in the admin sidebar to
be a section instead, which then shows all enabled plugin
admin routes (which are custom routes some plugins e.g.
chat define).

This is done via adding some special preloaded data for
all controllers based on AdminController, and also specifically
on Admin::PluginsController, to have the routes loaded without
additional requests on page load.

We just use a cog for all the route icons for now...we don't
have anything better.

spec/system/admin_revamp_sidebar_navigation_spec.rb

@@ -20,11 +20,14 @@ describe "Admin Revamp | Sidebar Navigation", type: :system do
     expect(sidebar).to have_no_section("admin-nav-section-root")
   end
 
-  it "does not show the admin sidebar if the setting is disabled" do
-    SiteSetting.enable_admin_sidebar_navigation = false
-    visit("/latest")
-    sidebar.click_link_in_section("community", "admin")
-    expect(page).to have_current_path("/admin")
-    expect(sidebar).to have_no_section("admin-nav-section-root")
+  context "when the setting is disabled" do
+    before { SiteSetting.enable_admin_sidebar_navigation = false }
+
+    it "does not show the admin sidebar" do
+      visit("/latest")
+      sidebar.click_link_in_section("community", "admin")
+      expect(page).to have_current_path("/admin")
+      expect(sidebar).to have_no_section("admin-nav-section-root")
+    end
   end
 end