created item groups and link banners and their associations and fixed bugs with join tables

back-office/test/models/item_group_test.rb

@@ -1,6 +1,6 @@
 require 'test_helper'
 
-class SpecialOfferTest < ActiveSupport::TestCase
+class ItemGroupTest < ActiveSupport::TestCase
   # test "the truth" do
   #   assert true
   # end

back-office/test/models/link_banner_test.rb

@@ -0,0 +1,7 @@
+require 'test_helper'
+
+class LinkBannerTest < ActiveSupport::TestCase
+  # test "the truth" do
+  #   assert true
+  # end
+end