Common HTML snippets / templates to be included in other templates

[waza-ari]

Is your feature request related to a problem? Please describe.

Many of the mails we're sending share common elements, e.g. a common footer containing required legal data, the mail footer, and so on. As of today, when using transactional mails, this part needs to be repeated across all templates.

Describe the solution you'd like

Have the option to include templates in each other, such that we can define a common footer template which then can be included in other templates. This would allow a lot of flexibility composing mails without repeating ourselves.