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
{F236077}
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D10884