Cannot find user_id from session

Hi @zainosaurus, 

I set everything up, but after all of that, it couldn't find the user_id from the session when I link my Spotify account.

```
File "/app/app/server.py", line 24, in load_user
return User(id = session['user_id']).find()
```

But my user shows up in Firestore.. 

Haven't looked at the code yet, just thought i'd let you know! 