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

src/docs/userguide/maniphest_custom.diviner

@@ -54,6 +54,8 @@ the field. These options are available:
     show next to the checkbox.
   - **checkbox-value**: If type is set to **bool**, the value to show on
     the detail view when the checkbox is selected.
+  - **default**: Default field value. For **date**, you can use a string like
+    `"July 4, 1990"`, `"5PM today"`, or any other valid input to `strtotime()`.
   - **copy**: When a user creates a task, the UI gives them an option to
     "Create Another Similar Task". Some fields from the original task are copied
     into the new task, while others are not; by default, fields are not copied.