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