Update timeline hyperlink colors
Summary: Makes hyperlinks in phui-timeline-title one color darker, with objects in bold.
Test Plan: Tested a timeline with people, objects, and timestamps.
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Differential Revision: https://secure.phabricator.com/D8214