Summary

This PR adds the wp-polyfill & wp-url JS polyfills so that the paired browsing JS files can be loaded on WP 4.9.

The paired browsing app was not being loaded due to the dependencies wp-polyfill & wp-urlbeing missing, and the client was missing the wp-polyfill dependency.

Fixes #3932

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).