Generate a default "Backlog" column for boards and put all tasks into it
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
Maniphest Tasks: T1344
Differential Revision: https://secure.phabricator.com/D7938