Define standard Email Template ID

We defined a *standard* Mail Template in Mailjet, which has a header image, footer etc. But all system mails and other mails which came from other modules (rest_password_reset for example) are still plain text emails, which went through the mailjet api still. It would be nice to set up a Template ID as default, which acts as a fallback here. Thus system mails would still have a template.

I guess, we can achieve the same with hook_mail() and checking if a Template ID is set or so, but for the nicer UX a UI way would be nice as well.