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