#18 Add sql index for unpublished exercises

And remove spec for remove feature

spec/factories/study_group.rb

@@ -0,0 +1,8 @@
+FactoryBot.define do
+  factory :study_group, class: StudyGroup do
+    association :consumer
+    sequence :name do |n|
+      "TestGroup#{n}"
+    end
+  end
+end