Continue lifting column layout logic out of ColumnPositionQuery

Summary:
Ref T10010. See D15174. This gets rid of the "actually apply the change" callsite and moves it to layout engine.

Next up is to make the board view use the layout engine, then throw away all the whack code in ColumnPositionQuery, then move forward with D15171.

Test Plan:

• Dragged tasks within a column.
• Dragged tasks between columns.
• Dragged tasks to empty columns.
• Created a task in a column.
• Swapped board to priority sort, dragged a bunch of stuff all over.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D15175