EmailMessage had a number of parameters that were required, like an HTML
body, that a caller may not have wanted to provide. Like Django's
EmailMessage, all parameters are now optional.

It also had some custom handling for the From field, added in
648eeaa377ab819b5a193e74241f529edf26a260, which was designed to prevent
some SPF issues. Looking into this further, best I can tell, it appears
that this code didn't actually solve any problems, but rather just
provided another way of setting the same information (unless a caller
wanted to access the "from_email" field on the attribute and get a
different value from what's in the headers, but it doesn't appear we do
this). That custom logic has been removed.

A handful of new unit tests have been added to test functionality, and
existing unit tests revised for improved naming and more specific test
coverage.

Unit tests in Djblets and Review Board pass.
Looked at some sample e-mail message headers, checking that they looked

sane.