Add "Stop Tracking" link to entries in the Phrequent search view.

Summary: Depends on D7163.  This adds a "Stop Tracking" link to the right-hand side of ongoing entries in the Phrequent search view.  It allows users to stop tracking items without first navigating to the item itself.

Test Plan: Started tracking and item and then clicked the "Stop Tracking" link in Phrequent.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3870

Differential Revision: https://secure.phabricator.com/D7164