Remove inconsistent and confusing use of the term "multiplex" in mail

Summary:
Ref T13053. Because I previously misunderstood what "multiplex" means, I used it in various contradictory and inconsistent ways.

We can send mail in two ways: either one mail to everyone with a big "To" and a big "Cc" (not default; better for mailing lists) or one mail to each recipient with just them in "To" (default; better for almost everything else).

"Multiplexing" is combining multiple signals over a single channel, so it more accurately describes the big to/cc. However, it is sometimes used to descibe the other approach. Since it's ambiguous and I've tainted it through misuse, get rid of it and use more clear language.

(There's still some likely misuse in the SMS stuff, and a couple of legitimate uses in other contexts.)

Test Plan: Grepped for multiplex, saw less of it.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D18994