In Task Graphs, provide a parent/child hint and fix weird strikethrough

Summary:
Fixes T11386. Ref T4788.

• Apparently fix weird strikethrough effect? Spooky!
• Provide a little icon hint in the left column about which tasks are direct parents/children, vs just reachable somehow. I don't think this is super useful/important, but seems maybe nice?

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788, T11386

Differential Revision: https://secure.phabricator.com/D16342