Fix an issue with creating tasks directly into milestone columns

Summary:
These columns were conflating projectPHID (the defualt project to add to the task) with boardPHID (the board the column appears on).

Separate them to fix the beahvior.

Test Plan: Used "Add Task" from dropdown menu of a milestone column on a parent project's workboard.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15242