Focus the message field when conversation opens

Fixes #317

// FREEBIE

js/views/conversation_view.js

@@ -25,6 +25,7 @@
             this.listenTo(this.model, 'change:name', this.updateTitle);
             this.listenTo(this.model, 'newmessages', this.fetchMessages);
             this.listenTo(this.model, 'change:unreadCount', this.onUnread);
+            this.listenTo(this.model, 'opened', this.focusMessageField);
 
             this.render();
             twemoji.parse(this.el, { base: '/images/twemoji/', size: 16 });