R1:3f56ca681f3e

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…