R1:66ff6d4a2ca0

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…