Email preview: WIP
Remove useless properties
PreviewMessage new extends EmailMultiRelated and provides additional
methods.
You can now override the message_class used to preview your email.
Add docstrings.
Added prefix for unicode strings in demo settings.py
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.
Remove useless import in previews
minor rewrites, relative imports, doc for email previewing
merge master in #13 from @thoas