title: Verification code rather than url in loopback
body: <p>I am using Loopback as an api for a phone app. </p>
  
  <p>How could I change the out of the box verification of an email address when a user signs up. I need it to be a 4 digit code (instead of a url) so that it's more friendly to the user verifying their account inside the app.</p>
  
  <p>ie. they would then just need to enter the 4 digit number to confirm the registration</p>