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