amp-iframe, amp-instagram, amp-youtube, amp-soundcloud wrapped in unnecessary p tags

[jerclarke]

Bug Description

This is a continuation of #4358 where the same problem applied to amp-facebook and was fixed.

The premise is that these amp-* objects should be in the root of the document, and not be wrapped in <p> tags. Twitter already worked this way, and in #4358 amp-facebook was fixed to match.

I've now noticed that amp-iframe, amp-instagram, amp-youtube, amp-soundcloud face the same problem:

As with the previous ticket, it's important to me because I need to style something that immediately follows the object (amp-instagram+.translation), but also generally it just seems like good practice all around.

Instagram embed code:

[embed]https://www.instagram.com/p/Buq8vU6Hcxn/[/embed]
<blockquote class="translation">This is a translation of the Instagram post above.</blockquote>

YouTube embed code

[embed]https://www.youtube.com/watch?v=vXPJVwwEmiM[/embed]
<blockquote class="translation">This is a translation of the YouTube video above.</blockquote>

Soundcloud embed code

[embed]https://soundcloud.com/globalvoices/summitpodcast[/embed]

Mostly likely this applies to other embeds as well. A general review would be greatly appreciated.

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