Add a "dateCreated" key to Maniphest
Summary: Depends on D6952. Unpunts there since I'm rolling into a swamp full of schema changes.
Test Plan: Issued date-constrained query and saw key as a candidate.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Differential Revision: https://secure.phabricator.com/D6954