Fix tab completion by moving whos_here() out of helpers

deb7f72cb6 ("Use Template#helpers method to define helpers.") moved
some methods away from being on the Template object to being Spacebars
helpers.  But Template.messages_input.events was making a call to
Template.chat_header.whos_here() directly, and that doesn't work
after the refactor.

There wasn't any need for the cross-template method call anyway,
so just make whos_here() a utility function of chat.coffee to fix.