R1:24a6eeb8d8a5

Allow the workboard backlog column to be reordered

Summary:
Fixes T5677.

- Instead of using `sequence == 0` to mean "this is the backlog column", flag the column explicitly.
- Migrate existing sequence 0 columns to have the flag.
- Add the flag when initializing or copying a board.
- Remove special backlog logic when reordering columns.

Test Plan:
- Migrated columns, viewed some boards, they looked identical.
- Reordered the backlog column a bunch of times (first, last, middle, dragged other…