Move board relationships to dedicated storage

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