add guardian email field to the appearance release public signing form

app/models/appearance_release.rb

@@ -39,6 +39,7 @@ class AppearanceRelease < ApplicationRecord
   # These validations apply to releases being signed by a minor
   with_options if: :minor? do
     validates :guardian_first_name, :guardian_last_name, presence: true
+    validates :guardian_email, email: true, allow_blank: true
   end
 
   validates :person_photo, content_type: face_photo_acceptable_content_types