message: Only add the code copy button for HTML messages
  Trivial fast-path optimisation: plain text messages cannot possibly contain pre
  blocks so there's no point in trying to parse them in order to add code copy
  buttons.
diff: @@ -393,7 +393,7 @@ export function bodyToHtml(content, highlights, opts) {
  }
  safeBody = sanitizeHtml(body, sanitizeHtmlParams);
  safeBody = unicodeToImage(safeBody);
  - safeBody = addCodeCopyButton(safeBody);
  + if (isHtml) safeBody = addCodeCopyButton(safeBody);
  }
  finally {
  delete sanitizeHtmlParams.textFilter;
  