Infinite Scroll: Pinterest embeds fail to load after initial page load

#### Steps to reproduce the issue

1) Activate any theme with Infinite Scroll support - we've tested Dara, Karuna, and Twenty Fifteen.
2) Add several posts (at least 8 to trigger infinite scrolling with multiple pages) with Pinterest embeds in each.
3) View the blog and scroll through the posts. Any posts loaded after page 1 will show an empty space where the Pinterest embed would be.

#### What I expected

Pinterest embeds should display in all posts.

#### What happened instead

Pinterest embeds aren't shown beyond the first page of posts. Likely similar to #4033 #7445 and others.

Originally reported by @ehti in Automattic/themes#12

* User report: 1302114-t
* User's site: https://jenniferkateevans.wordpress.com/category/year-2/year-2-research-file/brand-research/

Start by reproducing the issue with a supported Infinite Scroll theme and posts containing Pinterest embeds, then inspect how embeds are handled for posts loaded after the first page. Done means Pinterest embeds display in every post, including content appended by infinite scrolling.

Written by the indexing model from the issue text.