In Maniphest, don't render the task graph drawing if we're only showing parents/children

Summary:
Ref T4788. I thought I implemented this, but actualy didn't.

When we're in the "mid-sized" fallback mode (graph has more than 100 nodes, but not more than than 100 parents/children), don't actually draw the graph. It's almost always uninteresting and huge.

Instead, this just renders a list of direct parents, then the task, then the direct children, which is pretty straightforward.

Test Plan: Set limit to 5, saw mid-sized fallback graph with no actual graph drawing.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16816