Move form components into the form directory, and add a flash message.

app/src/frontend/components/flash-message.tsx

@@ -0,0 +1,7 @@
+const FlashMessage = (props: { message: string, severity?: string }) => {
+  return (
+    <div class="flash-message {props.severity ?? null}">
+      <span>{props.message}</span>
+    </div>
+  )
+};