Add StudyGroup to submissions and show it for submissions and RfCs

Also take care of deleting a StudyGroup for existing submissions

app/policies/submission_policy.rb

@@ -12,14 +12,8 @@ class SubmissionPolicy < ApplicationPolicy
     admin?
   end
 
-  def everyone_in_study_group
-    users_in_same_study_group = @record.study_groups.users
-    users_in_same_study_group.include? @user
-  end
-  private :everyone_in_study_group
 
-  def teacher_in_study_group
-    teacher? && everyone_in_study_group
+  def show_study_group?
+    admin? || teacher_in_study_group
   end
-  private :teacher_in_study_group
 end