Add invitation stats

Show some stats about unused invitations and active inviters

app/controllers/admin/invitations_controller.rb

@@ -1,6 +1,8 @@
 class Admin::InvitationsController < Admin::BaseController
   def index
     @current_section = :invitations
+    @invitations_unused_count = Invitation.unused.count
+    @users_with_referrals_count = Invitation.used.distinct.count(:user_id)
     @invitations_used = Invitation.used.order('used_at desc')
   end
 end