attempt 3 at fixing conpherence mail handler

Summary: you have to attach files and participants before you can start editing the conpherenece

Test Plan: push it live and try again

Reviewers: chad, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2399

Differential Revision: https://secure.phabricator.com/D4674