Add StudyGroups with ExternalUsers only

db/migrate/20181122090244_add_study_group_to_submission.rb

@@ -0,0 +1,5 @@
+class AddStudyGroupToSubmission < ActiveRecord::Migration[5.2]
+  def change
+    add_reference :submissions, :study_group, index: true, null: true, foreign_key: true
+  end
+end