Conpherence - fix conpherence create

Summary: somewhere along the line this broke. Before this patch we fail the visibility check since its based on Conpherence Participants which don't get created and attached until applyExternalEffects. Believe it or not, this was the least gross fix I could come up with; since the permission check is done SO early most other ideas I had involved creating a dummy participant object to pass the check then handling things for real later on... Ref T3723.

Test Plan: created a conpherence with myself - great success

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, Korvin, aran

Maniphest Tasks: T3723

Differential Revision: https://secure.phabricator.com/D6762