Implemented automatic end timers using new UI.

Summary:
This is a new implementation of the automatic
end timer functionality on top of the new UI.

See D5484 for a general overview of automatic
end timers.

Test Plan: See D5484 for the test plan.

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5577