This prevents users from activating automatic emails from listing pages
that are not valid.
This also adds more checks for the content of a newsletter, requiring
that a newsletter have at least one content block in order to be valid.
This change makes the server side validation check match what we're
checking in the editor in mailpoet/assets/js/src/newsletter_editor
/components/save.js and mailpoet/assets/js/src/newsletters/send.jsx
[MAILPOET-4236]
