Fix three minor edge case behaviors in Conpherence

Summary:
Couple of tweaks:

  - If a conpherence has no participants, we fail to `attachParticipants()`. This can happen if you leave a Conpherence as the last participant, then visit the URI again explicitly.
  - If you can't load any transactions (usually, because you don't have permission to view a thread's transactions), we try to attach `null` instead of `array()`. This can happen if you attempt to view a thread you don't have permission to see. A more general fix would be to tweak the load/filtering order, but I'm leaving that for another time since it's more involved and only gives us a small performance gain in unusual sitautions.
  - `initializeNewThread()` should be declared `static`.

Test Plan:
  - Viewed a thread with no participants, got proper policy error.
  - Viewed a thread I couldn't see, got proper policy error.
  - Grepped for `initializeNewThread()`.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8467