Generate a default "Backlog" column for boards and put all tasks into it

Summary:
Ref T1344. Autogenerates a "Backlog" column if one does not exist. Assigns all tasks to the backlog column.

For now, this column is always called "Backlog", but we could let it be called other things later.

Test Plan: Loaded a project, got a backlog column, created some columns.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1344

Differential Revision: https://secure.phabricator.com/D7938