R1:6db97bde127d

Build separate mail for each recipient, honoring recipient access levels

Summary:
Ref T6367. Removes `multiplexMail()`!

We can't pass a single body into a function which splits it anymore: we need to split recipients first, then build bodies for each recipient list. This lets us build separate bodies for each recipient's individual translation/access levels.

The new logic does this:

- First, split recipients into groups called "targets".
- Each target corresponds to one actual mail we're going to…