User sessions across devices

[jakerella]

When a user logs into multiple devices their user data in the redis store can easily be overwritten. To reproduce:

1. log in on device 1
2. do some stuff to get points (multiple things)
3. log in on device 2 (do not change device 1)
4. do a few things on device 2 (note that this is additive with what you did on device 1)
5. go back to device 1 (should still be logged in) and do something else to get points (a new thing)
6. notice that your score and inventory, etc only reflect what was done on device 1, nothing from device 2
7. go back to device 2 (still logged in) and refresh the page
8. notice that any progress on device 2 is lost

We should either restrict log ins to a single device (log out the user if they try to log in from a different device) or do a re-hydration of user data (or merge of data vs overwritten save?).