Facebox chat link with guest / login options
Update chat.html.haml
Restore use of initializer to define CHAT_*

Environment variables and Rails views are at two opposite ends
of the system: one very close to the environment, and the other
very close to the "outputs". It probably does not make a large
difference whether we wrap them or not in practice, but it
seems better to avoid referencing ENV vars directly in
views if we don't have to.
Add some copy explaining how this chat thing works, and tweak styling
Merge branch 'master' into chat
Clean up styling of chatbox
Merge branch 'chat'