Allow standard date fields to read default dates as strings

Summary: Ref T2217. Fixes T3866. We need to do a little more massaging before we can set strings as the value on datetime controls.

Test Plan: Set default to "1:23 AM", "2001/02/03".

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217, T3866

Differential Revision: https://secure.phabricator.com/D7116