#18 Add sql index for unpublished exercises

And remove spec for remove feature

db/migrate/20201208132844_add_index_to_unpublished_exercises.rb

@@ -0,0 +1,5 @@
+class AddIndexToUnpublishedExercises < ActiveRecord::Migration[5.2]
+  def change
+    add_index(:exercises, :id, where: 'NOT unpublished', name: :index_unpublished_exercises)
+  end
+end