fix(governance): reinstate deleted mock

In #4213, I deleted this mock because using `yarn run jest` reported
this as mocked twice. Now tests are failing because this isn't mocked.

- Reinstate deleted mock

apps/governance/src/__mocks__/react-markdown.js

@@ -0,0 +1,5 @@
+function ReactMarkdown({ children }) {
+  return <div>{children}</div>;
+}
+
+export default ReactMarkdown;