Fix an issue with creating tasks directly into workboard columns

Summary:
See https://discourse.phabricator-community.org/t/tasks-created-via-workboard-column-menu-are-moved-to-wrong-column/2200. The recent setIsConduitOnly() / setIsFormField() change (in D19842) disrupted creating tasks directly into a column from the workboard UI.

This field is a form field, it just doesn't render a visible control.

Test Plan:

• Created a task directly into a workboard column. Before: column selection ignored. After: appeared in correct column.
• Used "move on workboard" comment action.
• Edited tasks; edited forms for tasks. Didn't observe any collateral damage (weird "Column" fields being present).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19870