This uses the `current_user` watch in the `UserStore` instead of looping
every 100ms in order to detect if the user had signed in.

We are changing this because we noticed it was causing the deterministic
executor in tests to never detect a "parking with nothing left to run"
situation.

This seems better in production as well, especially for users who never
sign in.

/cc @maxdeviant 

Release Notes:

- N/A

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>