add index on lti_parameters to speed up several routes

db/migrate/20180904115948_add_index_for_lti_parameters_external_user_id.rb

@@ -0,0 +1,5 @@
+class AddIndexForLtiParametersExternalUserId < ActiveRecord::Migration
+  def change
+    add_index :lti_parameters, :external_users_id
+  end
+end