add index to assignment override students on quiz_id

refs #CORE-1717

Change-Id: Ice83f0693b7ef7ea7df22b19f4c2bb5fa3a947c7
Reviewed-on: https://gerrit.instructure.com/160711
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20180814155556_add_index_to_assignment_override_students_on_quiz.rb

@@ -0,0 +1,8 @@
+class AddIndexToAssignmentOverrideStudentsOnQuiz < ActiveRecord::Migration[5.1]
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def change
+    add_index :assignment_override_students, [:user_id, :quiz_id], :algorithm => :concurrently
+  end
+end