 Mixin used to edit and render some fields used when sending emails or
    notifications based on a mail template.

    Main current purpose is to hide details related to subject and body computation
    and rendering based on a mail.template. It also give the base tools to control
    who is allowed to edit body, notably when dealing with templating language
    like inline_template or qweb.

    It is meant to evolve in a near future with upcoming support of qweb and fine
    grain control of rendering access.
    