Add email preview by langs: WIP
Render body_html when a mail contains a text/html representation
Return Mail instance on send method
Refactor with preview with a site container which contains PreviewEmail registered
If preview object doesn't exist don't show the 'View mail' container
Lazy load mail object from PreviewMail
Add attachments in create_email_msg
Multiple enhancements:
* Add previews for emails, inspired by django-mailviews
* Add lang parameter to `_render_part` and `get_template_part` to
  override current lang.
* Add previews to rendering templates