R1:e4690a385447

Fix an issue where newly created tasks could appear at the bottom of columns

Summary:
Ref T10349. At HEAD, if you create a task //on a board//, it floats to the top correctly.

If you create a task elsewhere and tag it with the board, you were subject to the whims of the layout engine and it would generally end up on the bottom.

Instead, make the rules consistent so that "virtual" positions (of tasks which haven't been committed to a particular position yet) still float to the top.

Test Plan…