Message updates fail when connection drops

The way message updates are handled currently has the consequence that when the connection is dropped (e.g. the computer is put to sleep or disconnected from the network), then when the connection is restored the message updates don't begin again.

Notification updates and room member updates do continue after a network dropout.

This is caused by the different methods of updating. Notification and room member updates are triggered by a repeated periodical updating AJAX call. Message updates are single-shot. They send one AJAX call to the server and then the response from the server queues up the next call. Thus if the one response never gets through, then the next call never gets queued.

This is done this way because the call back to the server needs to know the highest message ID, but we can work around this by just having the updates set a variable somewhere on the page.