Using Html::escape() means we can't send HTML e-mails

Problem/Motivation

group_notify_mail() is currently doing this:

  $message['body'][] = Html::escape($params['content']);
  $message['body'][] = Html::escape($params['link']);

That means that the emails being sent out are full of escaped HTML. If you 'View source' on a message, you see this:

Posted by: Admin Root

<article id="node-1839"  data-history-node-id="1839" data-quickedit-entity-id="node/1839" role="article" class="contextual-region" about="/private-audio/test-audio-august-13-2020">
  <div>dww was here</div>
  
      <h2><span data-quickedit-field-id="node/1839/title/en/group_notify_email" class="field field--name-title field--type-string field--label-hidden">Test audio - August 13, 2020</span>
</h2>
...

And the message appears like so in your inbox:

Posted by: Admin Root <article id="node-1839" data-history-node-id="1839" data-quickedit-entity-id="node/1839" role="article" class="contextual-region" about="/private-audio/test-audio-august-13-2020"> <div>dww was here</div> <h2><span data-quickedit-field-id="node/1839/title/en/group_notify_email" class="field field--name-title field--type-string field--label-hidden">Test audio - August 13, 2020</span> </h2>...

Calling this a 'major' bug, since it seems to break the fundamental feature of this module, and there seems to be no work-around other than patching it.

Steps to reproduce

1. Configure a node type to send emails.
2. Configure the group_notify_email view mode for this node type to include markup.
3. Generate a notification.
4. Look at the results.

Proposed resolution

Don't use Html::escape(). Rely on the fact that the text format on the nodes should already be configured to prevent unsafe markup, or that only folks we trust have access to create them. We don't need to escape the HTML again, we need to send it off to the mailer system just like we were sending it to a browser. Then the raw message source looks like this:

Posted by: Admin Root

<article id="node-1839" data-history-node-id="1839" data-quickedit-entity-id="node/1839" role="article" class="contextual-region" about="http://dev.breema.com/private-audio/test-audio-august-13-2020"><div>dww was here</div>
  
      <h2><span data-quickedit-field-id="node/1839/title/en/group_notify_email" class="field field--name-title field--type-string field--label-hidden">Test audio - August 13, 2020</span>
</h2>

And the e-mail looks like it's supposed to in your inbox (see screenshots below).

Remaining tasks

1. Agree this is the right move. I don't understand how/why it's been like this all along. Maybe I'm missing something?
2. Upload the patch.
3. Reviews / refinements.
4. Commit.

User interface changes

HTML emails will now actually contain renderable HTML, not escaped tags that end up looking like raw HTML.

Before

After

API changes

Nope.

Data model changes

N/A