Set content-transfer encoding for email messages to base64.

Django (version <= 1.6) globally overrides python's default Content-Transfer-Encoding for UTF-8 messages from base64 to 7bit. We set it to base64 again otherwise mail servers add newlines and exclamation marks (sendmail ) when a line is longer than 989 characters.

This only applies to the plaintext version though; the html alternative and any non-text attachments are always encoded with base64. So any client which displays the plaintext version will show the extra exclamation marks and newlines.

Btw this behaviour has been changed in the latest django: https://code.djangoproject.com/ticket/12422

Screenshots on the ticket.
LMS-1466

@brianhw @sarina Can you review this?