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/background.js

@@ -116,6 +116,9 @@
 
   function mapOldThemeToNew(theme) {
     switch (theme) {
+      case 'dark':
+      case 'light':
+        return theme;
       case 'android-dark':
         return 'dark';
       case 'android':