THis feature is inspired by Joanie. Add two new urls to render Emails
HTML and Text templates.
Developpers can render the email template they are working on. When necessary,
run make mails-build, and reload `_debug__/mail/hello_html`, it will re-render
the updated email template.
Also, I have copy/pasted one template extra tags from Joanie, which loads
bas64 string from static images. This code is necessary to render the dummy
template `hello.html`.
