R1:4c4707e46763

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.