Correct rendering of workboard column move stories when a single transaction performs moves on multiple boards

Summary:
See https://discourse.phabricator-community.org/t/unhandled-exception-rendering-maniphest-task/3234.

If a single transaction performs column moves on multiple different boards (which is permitted in the API), the rendering logic currently fails. Make it render properly.

Test Plan: {F7011464}

Differential Revision: https://secure.phabricator.com/D20901