Subscriptions: Email for new post does not contain image from Embed Block in body of the email

#### Steps to reproduce the issue

1. Subscribe to a blog as a Follower.
2. Publish a new post with some text and after the first paragraph of text, add an image using Embed block.
3. Publish the post.

#### What I expected

In the subscription email, I expected it to show that image which was added using Embed block.

#### What happened instead

It didn't show that image.

PS: It works fine though if that image was added using Image block but the user needs to use Embed block.

**Screenshots**
https://d.pr/i/Krzbng

Reported on #600148-hc

Reproduce the subscription email with a post containing an image in an Embed block, then compare it with the working Image block case. Trace the Jetpack subscription email handling for post content and verify that the embedded image appears in the generated email.

Written by the indexing model from the issue text.