Summary:
Fixes T5476. Using edges to store which objects are on which board columns ends up being pretty awkward. In particular, it makes T4807 very difficult to implement.
Introduce a dedicated `BoardColumnPosition` storage.
This doesn't affect ordering rules (T4807) yet: boards are still arranged by priority. We just read which tasks are on which columns out of a new table.
Test Plan:
- Migrated data, then viewed some boards. Saw exactly the same data.
- Dragged tasks from column to column.
- Created a task directly into a column.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5476
Differential Revision: https://secure.phabricator.com/D10160
