Store online sessions, instead of users
Small refactor of socket handler code
Use socket to store and retreive session from boards
Added messaging when people join/leave the board
Don't emit join message back to user who joined
Send and save heartbeat
Expire stale sessions after five minutes
Better messaging
Re-join board via heartbeat if session cannot be found
Merge 'join' and 'heartbeat' messages
Merge branch 'chore/track-user-sockets' into development