End Cap for Timeline

Summary: End-cap for timeline. Fixes T4438

Test Plan: Tested on a timeline with and without endcap.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: aran, epriestley, Korvin, chad, btrahan

Maniphest Tasks: T4438

Differential Revision: https://secure.phabricator.com/D8530