Fix: Djrill assumes message content type is text

If you followed the instructions at:
<https://docs.djangoproject.com/en/1.5/topics/email/#sending-alternative-content-types>,
djrill would send the body as 'text' instead of 'html', resulting in
ugly emails. This feature has been documented as far back as [Django 1.0](https://docs.djangoproject.com/en/1.0/topics/email/#sending-alternative-content-types).

This also fixes how [`auto_text`][auto_text] could be set, but couldn't do anything.
If you wanted to use `auto_text=True`, you would have to send a null
body and then `attach_alternative` your real body. I didn't add a test
case for this because they're like... orthogonal..., and gets into
behavior testing vs unit testing.

  [auto_text]: https://djrill.readthedocs.org/en/master/usage/sending_mail/#auto_text