Provide task closed date via Conduit API, data export pipeline, and in list UI
Summary:
Depends on D19037. Ref T4434. Adds closed date to maniphest.search and "Export Data".
When a task has been closed, show the closed date with a checkmark in the UI instead of the modified date.
Test Plan:
- Exported data to CSV, saw close information.
- Saw close information in /maniphest/.
- Queried for close information via maniphest.search.
Maniphest Tasks: T4434
Differential Revision: https://secure.phabricator.com/D19038