Dangerous placement of the Firebase configuration

[TheLukaszNs]

Firebase configuration is placed in the index.html file and is publically visible not only to the end user but also for all people visiting this repo:

<script>
  // Initialize Firebase
  var config = {
    apiKey: "AIzaSyCv0ELrK-dnBDHNABwtbs22o8UAaAE_FC0",
    authDomain: "openpub-41be7.firebaseapp.com",
    databaseURL: "https://openpub-41be7.firebaseio.com",
    projectId: "openpub-41be7",
    storageBucket: "openpub-41be7.appspot.com",
    messagingSenderId: "624812372332"
  };
  firebase.initializeApp(config);
</script>

Malicious user can use this config (apiKey, messagingSenderId, etc.) to perform rather dangerous operations to the app.
We should export this to another file and add its path to the .gitignore file.