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