When moving an object between workboard columns, require object edit permission

Summary:
Fixes T5204. Currently, to move an object (like a task) between columns on a workboard, you must be able to edit the project.

This doesn't map very well to real usage. Instead, require users be able to edit the object (e.g., the task).

(You still need to be able to edit the project to create columns, edit columns, etc.)

Test Plan: Moved stuff around on a project I could not edit.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5204

Differential Revision: https://secure.phabricator.com/D9720