Allow datetime inputs to be optional

Summary:
Fixes T3279. For ApplicationSearch (and in some other cases) I'd like users to be able to provide an optional date. This isn't currently possible.

Add a checkbox which disables or enables the input.

Test Plan: Used UIExample to enter dates. Used Calendar to enter dates.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3279

Differential Revision: https://secure.phabricator.com/D6082