[WIP] Add post embed support

[westonruter]

This is split out from #889 for #841. We need a few things to fully support post embeds, including:

• Move the contents of the blockquote.wp-embedded-content into the amp-iframe as the placeholder. Note that this must not include the blockquote itself, because wpautop will break it.
• Allow clicking inside the post iframe to cause _top navigation.
• Allow iframe self-resize in AMP: https://www.ampproject.org/docs/reference/components/amp-iframe#iframe-resizing
• Consider inlining the embeds and not using an iframe at all for embeds from elsewhere on the site.

See previously for some key considerations: #288

A core patch may be required here as well.

Closes #809.

[swissspidy]

@westonruter Shall we close this in favor of #3465?

[westonruter]

Yes. Thanks!