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