Add ability to preview an email in the post editor

[simison]

What

Add the ability to preview an email in the post editor

Project thread: peKye1-6z-p2

Why

When drafting a post that is going to be sent as an email to subscribers, users would like to see how the email looks before publishing.

How

Customers could preview with "preview email" action in the editor's preview dropdown:

To extend the above menu, we should make it possible in Gutenberg first.

The actual preview could happen in a few ways:

1. Send the email to user's email address, and inform them we did so. See MailerLite as an example, although they also allow customizing the email:

   

2. Render the email preview in a modal

3. Render the email preview in the editor canvas

4. Share private URL for pages/posts with anyone

Each approach needs a bit of exploration to see how complex the above options might be.

Listing some secondary considerations:

• Sending the email to preview in an email reader might be a nice additional feature even if we'd implement preview in the editor.
• Previewing in the editor might pave the way for a nice editing experience for adjusting email template styles, header and footer content later on.
• Important to consider that previewing and switching back to editing might be annoying if the experience is clunky and slow.

[simison]

Started on allowing extending the dropdown in Gutenberg: WordPress/gutenberg#50605

[simison]

Email preview endpoint D110864-code