Text version of email templates

The generation of the text version only uses the content of editable sections, as explained here Within text version of email, the unsubscribe link does not display? by @Justin Cooperman​.

As a result, any important information that is hard coded in a non editable section is lost. Typically, this is a problem for a "view as web page" or the "unsubscribe link" sections if we want to protect them from user's creativity.

This idea is that, when creating an email template, we should be able to also define the text version of the template, including all the non editable sections.

-Greg