Add a plain-text field into Mail Editor

Subscriptions adds the Comment field to the Mail Editor pages. ME does not know anything about that field, it's managed by subscriptions_mail.mail_edit.inc.

#2076827: Add plaintext field in the ME queue intends to add a second body field, so that we have one for the HTML template and one for the plain text template, and we can send HTML mail with plain text alternative. To make this work in Subscriptions, we need a second Comment field, too.