When a task graph has too much stuff, only show adjacent nodes (direct parents/children)

Summary:
Ref T4788. This gives us a new level of graceful degradation, so now we show:

• Zero through 100 connected tasks: whole graph.
• More than 100 connnected tasks, but fewer than 100 direct parents/subtasks: just parents and subtasks, with "..." to hint that the graph is cut off.
• More than 100 parents and children: just the "sorry, too much stuff" error message.

Test Plan: {F1740882}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16344