Improve the way the Cover Block is rendered in notification emails

#### Steps to reproduce the issue

1. Subscribe by email to your own site.
2. Create a new post on that site. Be sure to include a Cover Block on the post
3. Look at the email notifying the new post

#### What I expected
I expected the image in the cover block to be managed better. I realize that it is next to impossible to emulate the way the image looks on the cover block itself, but it should at least use up all available with (instead of showing scaled down, `?w=560` on top of grey background).
pened instead

**Screenshots**

Original post:

Email:

I would suggest we **send the background of the cover block as a regular image and show any embedded block AFTER the cover block**. Any content included inside the cover block is currently lost (the _This text is sitting on top of a cover block_ text in this case).

Reproduce the issue by subscribing to the site by email, publishing a post with a Cover Block, and inspecting the notification email alongside the linked screenshots. Trace how the Cover Block is rendered in notification emails; done means the image uses the available width and embedded Cover Block content is preserved after it.

Written by the indexing model from the issue text.