DEV: Add dedicated admin groups page (#32377)

Previously, clicking "Groups" on the admin dashboard would bring you to the public groups page. Historically, the public and admin actions have been mixed together on that page.

This is a bit of a frustrating experience when working on the admin dashboard, and also prevented us from adding a "Settings" tab for group-related site settings.

This PR adds an "admin groups" page, which is just an exact copy of the public groups index for now. This allows us to add the "Settings" tab, and lets us gradually work un disentangling the public- and admin parts of groups.

app/controllers/admin/groups_controller.rb

@@ -3,6 +3,9 @@
 class Admin::GroupsController < Admin::StaffController
   MAX_AUTO_MEMBERSHIP_DOMAINS_LOOKUP = 10
 
+  def index
+  end
+
   def create
     guardian.ensure_can_create_group!