Survive receipt of too much workboard column position information

Summary:
Fixes T11468. I was able to find a reproduction case for the issue, at least, described in my comments there.

When the server sends back position information for columns we aren't displaying, for whatever reason, just ignore that information.

Future work in T4900 to do more synchronization may revisit this.

Test Plan: See T11468#190177 for steps. Did that stuff with this patch, got a clean edit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11468

Differential Revision: https://secure.phabricator.com/D16397