title: Getting user token just using the email on server side (Firebase)
body: <p>My question is this: I´m trying to send a push notification to a group of devices, I have a node of all the users and their current Firebase tokens. 
  When a user sends a message to a group of devices, I read each value of each user email and get each token, then proceed to send for each token a message. </p>
  
  <p>I know that Firebase provide a method for susbcriptions, the thing is that is not a good idea to use that because maybe the users don´t want to be susbribed to the specific topic of the message. </p>
  
  <p>I know that there´s a method on the server where I get the user´s email by getting their user token, is there a way to use it the other way around (getting the user token using the email)?</p>