Templates - add option for default archive template

[thebootable]

Is your feature request related to a problem? Please describe. I use two different templates, one for e-mail and one for the archive. This allows me to e.g. add a top-bar to the archive and some buttons linking to my homepage etc. But with that I have to actively set the archive-template in every message, which is easy to forget.

Describe the solution you'd like Having the option to define a separate template as the default template for archived messages would make sure I can't forget that and help with consistency.