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