Update `lastSeen` on users
Improve `lastSeen` handling all over the place.
Handle join/part better, plus greying out after 15 minutes.
Improve rendering of chat stream.
Re-render the entire stream once the app is enabled, which corrects errors with messages being receibved out of order, and also "reclaims" messages that are yours, now that we know who you are (since you've already entered an email at that point).