Conpherence - fix add participant / remove participant form

Summary: Ref T6713. We were dropping latest transaction ID. This should fix the "easy" part of T6713.

Test Plan: tried to add participants and it worked! (removing participants only allows yourself, but that worked too.)

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6713

Differential Revision: https://secure.phabricator.com/D10952