Add "Oldest" and "Fixed Last 7d" columns to Maniphest reports

Summary: Part of a user feature request, see T994.

Test Plan: Looked at data in columns, seemed to line up with reality.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley, sandra

Maniphest Tasks: T994

Differential Revision: https://secure.phabricator.com/D1944