Implement Maniphest auxiliary field 'default' key

Summary: Fixes T404. Ref T2575. Allows default to be set for any field. Date defaults are interpreted by `strtotime()`. Other defaults are interpreted as expected.

Test Plan:
  - Created a string custom field with default value "Orange".
  - Created a date custom field with a fixed default value (my birthday).
  - Created a date custom field with a relative default value ("today 4:59 PM").
  - Created/edited tasks with these fields, verified everything behaved sensibly.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T404, T2575

Differential Revision: https://secure.phabricator.com/D5282