Stop hard-coding any content in the email, rely on the 'group_notify_email' view mode

Problem/Motivation

Currently, group_notify is injecting some stuff into the e-mail body directly in group_notify_mail() and group_notify_notify(). There's all the nice plumbing of a separate view mode for what you want to see in your emails, and a twig template, but these items are hard-coded and you're stuck with them:

• 'Posted by: @author'
• 'View full post' link.

Proposed resolution

Only put the 'content' key in the message body (the rendered node or comment), and let sites configure their view mode and twig templates accordingly if they these things.

Remaining tasks

1. Agree if this is wise.
2. Upload the fix.
3. Figure out if / how we want to warn folks during the upgrade that these items are disappearing and if they want them, they need to update their view mode config accordingly.
4. Reviews / refinements.
5. Commit.

User interface changes

Nothing hard-coded in email notifications. Everything is controlled by the 'group_notify_email' view modes on nodes and comments.

API changes

Not really. ;)

Data model changes

Sort of.