The layout class is the only class that should have knowledge of
page-level constant markup, such as #gutter and #contacts, and
should be pretty much the only place we find elements by id (with
the exception of template elements).
This change removes references to #gutter from views. Rather than
hardcoding assumptions about page layout, view elements should
ask the layout to insert themselves into the main content area by
calling Whisper.Layout.setContent.
Each conversation views now manages its own separate elements
rather than all binding to a shared #conversation element, and
similarly for message composition ui.
Also includes the beginnings of group creation UI (not working yet),
featuring bootstrap-tagsinput field for entering group recipients
Let ConversationListItemView save a reference to its corresponding
ConversationView. This lets it render or delegate/undelegate events
when opening and closing a conversation.
Similarly for ConversationView itself, which contains a MessageListView.
