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