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/last_seen_indicator_view.js

@@ -7,7 +7,7 @@
   window.Whisper = window.Whisper || {};
 
   Whisper.LastSeenIndicatorView = Whisper.View.extend({
-    className: 'last-seen-indicator-view',
+    className: 'module-last-seen-indicator',
     templateName: 'last-seen-indicator-view',
     initialize(options = {}) {
       this.count = options.count || 0;