perf(chat): Stop sending nickname with every message to the server.

Since the server knows the nickname of the sender, apply it at server side. This also prevents a potential 'exploit' of faking the sender.
fix(chat): Use ng-cloak to prevent issues while loading.

Previously, loading would display angular expressions prior to finishing loading.
feat(chat): Allow - and _ in usernames and disallow the name "system" in any form.
fix(chat): Disallow _ in usernames since it's used in markdown.