Add ReallyMajorEvent to PHUITimelineView

Summary: For actions like "Close" that are in theory stopping the timeline, we should display some disruption to the line itself.

Test Plan:
Tested in UIExamples

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10884