Create subscription for receiving updates on a user's account

Summary

We rely on some pieces of user info to be up-to-date in the KV store, namely the user's email address. If the email changes in Microsoft's system, it needs to be updated in our system.

The task here is to create a subscription on each connected user to receive updates on the user's account. Then update the KV store to reflect these changes.

Alternatively, we can have the users reconnect their account when their email is changed in Microsoft's system. cc @levb