[Redesign] Clean up timeline images

Summary: Ref T8099, Timeline images should be 28 and 50, I previously had a border adding the additonal size, but changed that to an inset shadow.

Test Plan: Staring. Intensifies.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13368