Support "date" custom fields in "*.edit" endpoints
Summary: Fixes T13355. This didn't appear to be a ton of extra work, we just didn't get it for free in the original implementation in D14635.
Test Plan:
- Saw "date" custom fields appear in Conduit API documentation for "maniphest.edit".
- Set custom "date" field to null and non-null values via the API.
Maniphest Tasks: T13355
Differential Revision: https://secure.phabricator.com/D20690