[BUGFIX] Digest (feeds) email creation failing

Digest (feed) are failing to create due to a recent change in the email creation consumer method ( !113 (diffs) ).

https://push-notifications-sentry.web.cern.ch/sentry/consumers/issues/2668/

Email feed processor uses the same create_email method from email/utils which now modifies context["message_body"]. This does not work when called from email_feed/processor.

The context["message_body"] logic should then be moved to the email/processor before calling create_email.