Exclude closed tasks from blocked/blocking filter

Summary: Fixes T7392. When filtering blocked/blocking Maniphest tasks, don't consider closed tasks.

Test Plan:

1. Created T1 and T2 with T2 depends on T1.
2. Marked T1 as resolved.
3. Searched for tasks "blocked by other tasks" and noted that T2 wasn't in the result set.

Reviewers: btrahan, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7392

Differential Revision: https://secure.phabricator.com/D11911