Add lang parameter to get_template_part
Add `get_mail_preview` to retrieve a PreviewMail class.
Make `get_mail_class` and `get_mail_object` public.
Remove `body_html_escaped`.
Add rendering filtered by formats.
Fix tests: replace `get_mail_class` by `get_mail_class`.
Add MailPreviewMessageView to output HTML format.
Use mail_factory_preview_message for iframe.
Add template for MailPreviewMessageView.
Add french version for comments mail in demo.