R1:a372627fcd54

Provide URI/API support for querying subtasks/parents of a particular task

Summary:
Ref T8126. Ref T4788. This adds a way to query by parent or subtask.

I plan to link to this from the task graph (e.g., {nav View > Search Subtasks} or similar, in a dropdown on the "Task Graph" element) as a way to let us bail out if tasks have 300 subtasks and send the user to a big query result list. That'll give us more flexibility to tailor the UI for reasonable numbers of tasks.

There's no UI for this unless you…