Conpherence - get back-end prepped for loading less transactions all the time

Summary: this just does the back-end migration. I realized that we don't need to keep track of cacheTitle and cachePhoto since those are based off recent participation handles and dynamic relative to who is viewing it. Also kept the "last seen phid" as I think that will be useful to have auto-scroll to where you last read.  Ref T2867.

Test Plan: did the migration. observed sensical values in the database. created a new conpherence - again sensical values. updated a conpherence - more sensical values.

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T2867

Differential Revision: https://secure.phabricator.com/D5567