More resilient timeline icon layout

Summary: center aligns the icons in the fill area, removes some of the positioning jank. Also set new icons for maniphest custom.

Test Plan: test desktop and mobile layouts, tested thin pins for proper centering.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4866

Differential Revision: https://secure.phabricator.com/D8839