R1:314331095411

Use pixel as background of timeline

Summary: Makes the line in timeline a background "image" instead.

Test Plan: Tested Desktop, Mobile, and Tablet breakpoints with lots of content.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8221