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

ts/components/conversation/Emojify.tsx

@@ -34,10 +34,13 @@ function getImageTag({
   const title = getTitle(result.value);
 
   return (
+    // tslint:disable-next-line react-a11y-img-has-alt
     <img
       key={key}
       src={img.path}
-      alt={i18n('emojiAlt', [title || ''])}
+      // We can't use alt or it will be what is captured when a user copies message
+      //   contents ("Emoji of ':1'"). Instead, we want the title to be copied (':+1:').
+      aria-label={i18n('emojiAlt', [title || ''])}
       className={classNames('emoji', sizeClass)}
       data-codepoints={img.full_idx}
       title={`:${title}:`}