Allow "Assign/Claim" stacked action on closed tasks

Summary: Ref T9908. Now that you can submit multiple actions, you can "Open + Assign/Claim" a closed task, which is a reasonable action.

Test Plan: Assign/claim'd a closed task.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9908

Differential Revision: https://secure.phabricator.com/D14692