Add sanitization of XFBML raw embeds into amp-facebook

[westonruter]

Summary

I'm just opening this PR as a potential starting point for converting <fb:post> into <amp-facebook>, where fb:post is the XFBML version of what Facebook now has replaced with div.fb-post. Jetpack was using fb:post which is why this issue was discovered, but I've opened Automattic/jetpack#17492 to update Jetpack to use div.fb-post instead. Once that is released, this PR should be very low priority since XFBML is not the current best practice.

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).

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