[FIX] email_template: default model for save as new template feature

When sending a message with the "Compose new message" button on the right of the user menu, in the top bar, if you tried to save the message as template, you had a traceback because model field of email.template is mandatory, but was set to True because there is no model in such a case.

As there is no any relevant model in such a place, and that the field is mandatory, mail.message is pretty convenient as the default value.