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

test/i18n_test.js

@@ -12,7 +12,10 @@ describe('i18n', function() {
     });
     it('returns message with multiple substitutions', function() {
       const actual = i18n('theyChangedTheTimer', ['Someone', '5 minutes']);
-      assert.equal(actual, 'Someone set the timer to 5 minutes.');
+      assert.equal(
+        actual,
+        'Someone set the disappearing message timer to 5 minutes'
+      );
     });
   });