Overwriting HTML emails impossible when using solidus_digital

[fschwahn]

Currently solidus_digital only overwrites the text template of the confirm_email. We need to adjust this because it leads to quirky issues where overwriting the html template of a mailer is impossible without also overwriting the text template.

There are 2 solutions:

1. The parts about digital products are inserted into the confirm_email using deface. That's the preferable solution.
2. We also add the html version of confirm_email to the gem. We'd need to keep up changes in solidus in the confirm_email if we choose that route. (the email text is still from spree, so it would probably need updates right away)