Add LndHub db/models, and quick stats for admin views

app/controllers/admin/invitations_controller.rb

@@ -1,8 +1,11 @@
 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')
+    @stats = {
+      available: Invitation.unused.count,
+      accepted: @invitations_used.length,
+      users_with_referrals: Invitation.used.distinct.count(:user_id)
+    }
   end
 end