Fix "Blocked" task queries with multiple subtasks, and update language

Summary:
Ref T8126. See that task for discussion. This change:

• Updates language to be more consistent ("Parents", "Subtasks") since I moved us away from the often-confusing "Block" language in T4788.
• Fixes bugs with finding the wrong set of tasks if tasks have a mixture of open and closed subtasks or parents.

Test Plan:

• Created four tasks: no subtasks, one closed subtask, one open subtask, mixture of open and closed subtasks.
• Created four more tasks: no parents, one closed parent, one open parent, mixture of open and closed parents.
• Searched for all this stuff, got the proper results:

{F1740683}

{F1740684}

{F1740685}

{F1740686}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8126

Differential Revision: https://secure.phabricator.com/D16340