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 stuff around).
  - Added tasks to a project, saw them show up in the reordered backlog.
  - Initialized a new board and saw a backlog column show up.
  - Copied an existing board and saw the backlog column come over.
  - Tried to hide a backlog column.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5677

Differential Revision: https://secure.phabricator.com/D10189