Make Phrequent time accounting aware of the stack
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:
Test Plan: Added a bunch of unit tests, mucked around in the UI.
Reviewed By: btrahan
CC: hach-que, skyronic, aran
Maniphest Tasks: T3567, T3569
Differential Revision: https://secure.phabricator.com/D7349