Implemented showing the number of objects tracked as application status.

Summary:
Implementing that TODO where we want to show the current number of
objects being tracked by a user on the application icon so that they're
aware of any timers that are running.

Depends on D5479

Test Plan:
Apply this patch and track a Maniphest task. The counter should show
the number of objects you are tracking in the navigation pane of the
main screen

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T2857

Differential Revision: https://secure.phabricator.com/D5480