Bug: need to add client heartbeat timeouts to detect when users have shut the tab

[Aaronontheweb]

We are pretty good about automatically exiting users from sessions whenever they navigate away from a drawing session, but we need to also include a heartbeat system to detect when a user has closed a tab.

Normally we could handle this via WebSocket disposal events but since we're outsourcing that to the Blazor Hub we might need to write some code to explicitly check for this.