Summary:
Ref T3569. Fixes T3567. When figuring out how much time has been spent on an object, subtract "preemptive" events which interrupted the object.
Also, make the UI look vaguely sane:
{F72773}
Test Plan: Added a bunch of unit tests, mucked around in the UI.
Reviewers: btrahan
Reviewed By: btrahan
CC: hach-que, skyronic, aran
Maniphest Tasks: T3567, T3569
Differential Revision: https://secure.phabricator.com/D7349
