Twenty Seventeen: Double bottom-margins appear below embeds

[westonruter]

There is some CSS in Twenty Seventeen to make sure embeds and iframes fit their containers:

embed,
iframe,
object {
        margin-bottom: 1.5em;
        max-width: 100%;
}

However, in the case of an oEmbed where a video appears inside of its own paragraph the result is a double margin between the iframe and the next paragraph. The margin should only be added if the iframe is the only child of the parent element.

This is a problem both for videos in post content and for videos embedded into the Text widget via #40854.

[davidakennedy]

Tested this. I can confirm the bug, and the patch looks good to me. Thanks @westonruter!

[davidakennedy]

In 41888:

Twenty Seventeen: Fix double bottom-margins below embeds

Corrects an issue where both the containing paragraph and embed had bottom margins, causing unwanted extra space.

Props westonruter.

Fixes #42118.