Persist column membership after tasks are dragged around

Summary: Ref T1344. Write edges and read them when reloading the board.

Test Plan: After reload, stuff stays mostly where I put it. In-column order isn't always persisted correctly yet.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1344

Differential Revision: https://secure.phabricator.com/D7944