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
Maniphest Tasks: T1344
Differential Revision: https://secure.phabricator.com/D7944