Finish new Message component, integrate into application

Also:
- New schema version 8 with video/image thumbnails, screenshots, sizes
- Upgrade messages not at current schema version when loading messages
  to show in conversation
- New MessageDetail react component
- New ConversationHeader react component

js/views/inbox_view.js

@@ -108,7 +108,9 @@
       const inboxCollection = getInboxCollection();
 
       inboxCollection.on('messageError', () => {
-        this.networkStatusView.render();
+        if (this.networkStatusView) {
+          this.networkStatusView.render();
+        }
       });
 
       this.inboxListView = new Whisper.ConversationListView({