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.
