Add StudyGroup to submissions and show it for submissions and RfCs

Also take care of deleting a StudyGroup for existing submissions

app/controllers/concerns/lti.rb

@@ -168,6 +168,7 @@ module Lti
     end
     group.users |= [@current_user] # add current user if not already member of the group
     group.save
+    session[:study_group_id] = group.id
   end
 
   def set_embedding_options

app/controllers/concerns/submission_parameters.rb

@@ -16,7 +16,8 @@ module SubmissionParameters
       current_user_id = current_user.id
       current_user_class_name = current_user.class.name
     end
-    submission_params = params[:submission].present? ? params[:submission].permit(:cause, :exercise_id, files_attributes: file_attributes).merge(user_id: current_user_id, user_type: current_user_class_name) : {}
+    # The study_group_id might not be present in the session (e.g. for internal users), resulting in session[:study_group_id] = nil which is intended.
+    submission_params = params[:submission].present? ? params[:submission].permit(:cause, :exercise_id, files_attributes: file_attributes).merge(user_id: current_user_id, user_type: current_user_class_name, study_group_id: session[:study_group_id]) : {}
     reject_illegal_file_attributes!(submission_params)
     submission_params
   end