For now, hard limit task graph at 100 nodes

Summary:
Ref T4788. One install has some particularly impressive task graphs which are thousands of nodes large.

The current graph is pretty broken in these cases. For now, just render a "too big to show" message. In the future, I'd plan to finesse this (e.g., show parents/children, show links to parents/children, etc).

Test Plan:

• Viewed a normal task.
• Set limit to 3, viewed a task with graph size 6, saw an error message.
• Viewed a revision stack graph (unaffected).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16295