add audit logging to class member table

Closes #417

adds audit logging to class member table, using school ID meta field

Author: Conor

app/models/class_member.rb

@@ -12,6 +12,13 @@ class ClassMember < ApplicationRecord
 
   validate :student_has_the_school_student_role_for_the_school
 
+  has_paper_trail(
+    if: ->(cm) { cm.school_class&.school_id },
+    meta: {
+      meta_school_id: ->(cm) { cm.school_class&.school_id }
+    }
+  )
+
   private
 
   def student_has_the_school_student_role_for_the_school