Refactor LTI parameters and add study group

* This change also requires that submissions in our test need to have a valid study group.

app/models/lti_parameter.rb

@@ -1,9 +1,12 @@
 # frozen_string_literal: true
 
 class LtiParameter < ApplicationRecord
-  belongs_to :consumer, foreign_key: 'consumers_id'
-  belongs_to :exercise, foreign_key: 'exercises_id'
-  belongs_to :external_user, foreign_key: 'external_users_id'
+  belongs_to :exercise
+  belongs_to :external_user
+  belongs_to :study_group, optional: true
+  delegate :consumer, to: :external_user
+
+  validates :external_user_id, uniqueness: {scope: %i[study_group_id exercise_id]}
 
   scope :lis_outcome_service_url?, lambda {
     where("lti_parameters.lti_parameters ? 'lis_outcome_service_url'")