Add role to ExternalUser and update it via LTI

db/migrate/20181116143743_add_role_to_external_users.rb

@@ -0,0 +1,5 @@
+class AddRoleToExternalUsers < ActiveRecord::Migration[5.2]
+  def change
+    add_column :external_users, :role, :string, default: 'learner', null: false
+  end
+end