A previous change to the mail header code resulted in the From header
being included in e-mails twice, with different values. Gmail (and
likely other mail systems) check for this, and will error out if they
don't match.

This was happening because we changed the mail header code to no longer
pass any headers down to the EmailMessage constructor, opting instead to
inject headers ourselves. This is fine, except we did this to the From
header as well, which is considered special. The constructor needs to
know about this one, and since we no longer provided it, it fell back to
a default address. Our From header then got added later on, resulting in
two different headers added.

The solution is simple. We just make From special again, and pass its
header down to the constructor.

I've also renamed a member variable to help guard against any potential
future changes to Django's EmailMessage class.

Unit tests were added and updated to test for this.

Unit tests passed.
Viewed a copy of the sent e-mail in some manual testing. There was only

one From header, and it had the correct value. Previously, I could see

two conflicting values.