R1:c715b42f3612

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…