Manage date control enabled state as part of DateControlValue

Summary: Ref T8024. Allow DateControlValue to manage enabled/disabled state, so we can eventually delete the copy of this logic in DateControl.

Test Plan:

• Used Calendar ApplicationSearch queries to observe improved behaviors:
  • Error for invalid start date, if enabled.
  • Error for invalid end date, if enabled.
  • Error for invalid date range, if both enabled.
  • When submitting an invalid date (for example, with the time "Tea Time"), form retains invalid date verbatim instead of discarding information.
• Created an event, using existing date controls to check that I didn't break anything.

Reviewers: chad, lpriestley, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8024

Differential Revision: https://secure.phabricator.com/D12673