Change class User to old class name FirebaseUser

[doiya46]

class User is general for almost project, but when use with firebase_auth, the old version name is FirebaseUser. Now almost file we need to refactor like this import 'package:firebase_auth/firebase_auth.dart' as firebase_auth; and use firebase_auth.User, it's a bad name. And almost class need to change like

firebase_auth.FirebaseAuth.instance.signOut
firebase_auth.GoogleAuthProvider
firebase_auth.AuthCredential
.....
....

[ToniTornado]

I refactored my User class to be an AppUser to work around this.