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., 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 specify a query yourself, so the only ways to get to it are:

• Manually put ?parentIDs=... into the URI.
• Use the API.
• Future link from task graphs.

It doesn't seem too useful to me on its own, outside of the context of links from tasks.

Test Plan:

• Manually put ?parentIDs=... and ?subtaskIDs=... into Maniphest query UI, got expected results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788, T8126

Differential Revision: https://secure.phabricator.com/D16341