Add pagination to admin pages

app/controllers/admin/invitations_controller.rb

@@ -1,7 +1,8 @@
 class Admin::InvitationsController < Admin::BaseController
   def index
     @current_section = :invitations
-    @invitations_used = Invitation.used.order('used_at desc')
+    @pagy, @invitations_used = pagy(Invitation.used.order('used_at desc'))
+
     @stats = {
       available: Invitation.unused.count,
       accepted: @invitations_used.length,