R1:cac26c882476

Fix errant rules for associating projects when dragging tasks within a milestone column

Summary:
Fixes T10912. When you drag tasks within a milestone, we currently apply an overbroad, API-focused rule and add the parent board's project. This logic was added fairly recently, as part of T6027, to improve the behavior of API-originated moves.

Later on, this causes the task to toggle in and out of the parent project on every alternate drag.

This logic is also partially duplicated in the `MoveController`.