Base template or global header/footer for customer emails

[shoecake]

This feature request has been asked for before #599 and #531 in 2016 but were closed without comment.

I am finding myself modifying the same header/footer onto all email templates which seems time consuming and prone to errors. It would be great if I could just edit them in one place and it apply to all emails.

This could be implemented via a base template all emails inherit from or via new message tokens (maybe a token to insert a topic?) Than way common content could be managed in one place.