Support for multiple sessions

[lukas-shawford]

It should be possible to have multiple users (or the same user logged into multiple machines) edit the same quiz and see each other's changes without anything getting overwritten. Ideally, changes made in one session should be visible as close to real time as feasible in all other simultaneously connected sessions. If one client temporarily disconnects then reconnects, all changes made in all other sessions should get pushed to them before they are allowed to save anything. If conflicting changes are made, the UI needs to provide a way to merge/resolve these conflicts.

Related: Something similar may need to be done for topics. Perhaps there should be a common scheme for broadcasting all changes (to quizzes, topics, and whatever else might be added in the future) to all connected clients.

The implementation of this will likely be dependent on #5.