Back in the Review Board 2.5 development cycle, the old

"SpiffyEmailMessage class was split out into an EmailMessage in

Djblets, allowing us to use the same logic in other products. We then

later updated Review Board 3.0 to make use of this class, but we left

the version in 2.5 alone, given where we were in the release cycle.
Now that the class has been well-tested, and has functionality we want

in Review Board 2.5, we can safely migrate over to it. This change

backports the Review Board 3.0 change to use the Djblets EmailMessage. It

also updates newer code to use it, and updates all call sites to pass

keyword arguments instead of positional (given the addition of newer fields

in the class).
This will be needed before we can begin using DMARC support in Review Board.

Unit tests pass.