Notify original user email when the email changes

This adds a new setting `send_email_change_notification` which will
send an email to the original user email when their email is updated to
a new one.

It doesn't take into account the reconfirmable setting yet, it will be
added next, so that if confirmable is included and reconfirmable is
being used, the email will be triggered when the email change is
requested, not when confirmed (e.g when we store the email in
`unconfirmed_email`, not when it's later copied to `email` when that is
confirmed).

config/locales/en.yml

@@ -23,6 +23,8 @@ en:
         subject: "Reset password instructions"
       unlock_instructions:
         subject: "Unlock instructions"
+      email_change:
+        subject: "Email Changed"
       password_change:
         subject: "Password Changed"
     omniauth_callbacks: