Implement Maniphest "date" auxiliary field type
Summary:
Ref T404. Ref T2575. Adds a "date" type to Maniphest.
This doesn't let you default the date to anything other than time(); I'll do that in the next diff.
Test Plan: Created and edited a task with date fields.
Reviewers: hach-que, btrahan
Reviewed By: btrahan
CC: aran, mbishopim3
Maniphest Tasks: T404, T2575
Differential Revision: https://secure.phabricator.com/D5281