Allow filtering of "date" custom fields

Summary: Ref T4663. Ref T4659. Allows "date" fields to be filtered with range parameters.

Test Plan:
  - Added a custom "date" field with "search".
  - Populated some values.
  - Searched for dates using new range filters.
  - Combined date search with other searches.
  - Ran other searches independently.
  - Inspected the generated queries.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: shadowhand, epriestley

Maniphest Tasks: T4659, T4663

Differential Revision: https://secure.phabricator.com/D8598