Session chat cleanup.

Unify switch-statement using objects.
Do not parse recursively, but do a linear 2-step process. Remove msg.me property and move "/me" parsing out of that function.
Early return instead of using msg.hide.
Add observer-checks for private messages to impede abuse and fix #3270.
Remove the "/all" command, since it doesn't add functionality but complexity.
Remove a TODO as displaying multiple contexts ("/team /allies text"?) is not useful currently and made the previous code error-prone.