title: How to send unique registration links with firebase?
body: <p>I want to build an application where users just can register with "Invitation". Users already with account can send invitations to the specified e-mail addresses, then the app send an e-mail with a link to the registration where there's a token and just the specified e-mail address can be used. </p>
  
  <p>So first, how to an "automated" e-mail from firebase, secondly, how to generate an unique registration token to the URL?</p>