fix(app-root): allow search engines in production

- remove `/public/robots.txt`.
- add `/app/robots.js` which builds a dynamic `robots.txt` file, depending on `APP_ENV`. Allow access to `/` in production, but block the Zooniverse site otherwise.

packages/app-root/src/app/robots.js

@@ -0,0 +1,19 @@
+const appEnv = process.env.APP_ENV || "development";
+
+export default function robots() {
+  const isProduction = appEnv === "production";
+  if (!isProduction) {
+    return {
+      rules: {
+        userAgent: "*",
+        disallow: "/",
+      },
+    };
+  }
+  return {
+    rules: {
+      userAgent: "*",
+      allow: "/",
+    },
+  };
+}