Clear message collections when the window is closed

We don't need to keep them in memory if we're done viewing them,
plus it avoids having to re-render a large collection when we re-open a
conversation. Now that we only load a sensible number of messages at a
time, caching them between usages is less valuable. Removing them from
the collection should free them for garbage collection.

// FREEBIE

js/views/conversation_view.js

@@ -62,6 +62,7 @@
                 this.appWindow.contentWindow.removeEventListener('focus', onFocus);
                 window.autosize.destroy(this.$messageField);
                 this.remove();
+                this.model.messageCollection.reset([]);
             }.bind(this));
 
             this.fetchMessages();