Actually return participants for a thread.

Currently we're deduplicating only on the (display name, email) pair, so a
given thread may contain e.g. both

{ "email": "pika-private@mit.edu",
  "name": "\"pika-private@mit.edu\"\t" }

and

{ "email": "pika-private@mit.edu",
  "name": "Wrathful Artichokes" },

as separate participants, but this is better than nothing for now.