Mailchimp block: Handle embeds in Jetpack subscription emails

#### Steps to reproduce

- Start with a site with Jetpack subscriptions enabled
- Subscribe to the site with your email
- Add a Mailchimp block to a post/page
- Publish

#### What is expected

The Mailchimp block shown on the email is handled better: either is ignored, a placeholder appears, or shows the actual block

#### What happens instead

The block appears broken, and shows all the success/failure messages at the same time

#manual-testing

Start by reproducing the listed steps with Jetpack subscriptions enabled and a Mailchimp block in a published post or page. Trace how that block is rendered in the subscription email, then verify that it is ignored, replaced by a placeholder, or rendered correctly without showing all success and failure messages together.

Written by the indexing model from the issue text.