Make it possible to specify the plain text email template

[vberlier]

Prerequisites

• [ ] Is it a bug?
• [x] Is it a new feature?
• [ ] Is it a a question?
• [ ] Can you reproduce the problem?
• [x] Are you running the latest version?
• [x] Did you check for similar issues?
• [x] Did you perform a cursory search?

For more information, see the CONTRIBUTING guide.

Description

Right now the plain text version of the email is generated on the fly, it would be nice to be able to use a plain text template.