Render parent and child tasks in Maniphest with a graph trace

Summary: Ref T4788. This seems reasonable locally, but not sure how it will feel on real data. Might need some tweaks, or might just be a terrible idea.

Test Plan: {F1708059}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16214