All necessary AMP extensions should be loaded on the frontend so that a valid AMP story can be viewed.

This is a follow-up to https://github.com/ampproject/amp-wp/issues/3862.

QA Instructions

1. Verify that publishing a new story and viewing it on the frontend produces valid AMP, with the exception of a missing poster warning
2. Verify that adding a poster image removes that last warning

Can be easily tested with the AMP validator Chrome extension.