R1:eca7d3fedae5

Expand aggregate email recipients prior to multiplexing

Summary:
Ref T4361. Before we figure out which To/CC are addressable, try to expand To/CC. Specifically, the supported expansion right now is project PHIDs expanding to all their members.

Because of the way multiplexing works, we have to do this in two places: explicitly in `multiplexMail()`, and when sending mail that wasn't multiplexed. This is messy; eventually we can get rid of it (after ApplicationTransactions are everywhere).

This has some…