Add auditing to roles (#491)

## Status

- Related to
RaspberryPiFoundation/digital-editor-issues#404

## What's changed?

Adds auditing to roles.

## Steps to perform after deploying to production

N/A

Author: danhalson

app/models/role.rb

@@ -10,6 +10,12 @@ class Role < ApplicationRecord
   validate :students_cannot_have_additional_roles
   validate :users_can_only_have_roles_in_one_school
 
+  has_paper_trail(
+    meta: {
+      meta_school_id: ->(cm) { cm.school&.id }
+    }
+  )
+
   private
 
   def students_cannot_have_additional_roles

spec/models/role_spec.rb

@@ -3,7 +3,7 @@
 require 'rails_helper'
 
 RSpec.describe Role do
-  describe 'validations' do
+  describe 'validations', versioning: true do
     subject(:role) { build(:role) }
 
     it 'has a valid default factory' do
@@ -39,6 +39,11 @@
       expect(duplicate_role).to be_invalid
     end
 
+    it 'enables auditing' do
+      role.save
+      expect(role.versions.length).to(eq(1))
+    end
+
     context 'when the student role exists for a user and school' do
       let(:user) { build(:user) }
       let(:school) { build(:school) }