Automatically make timeline + objectbox sit flush

Summary: Right now you have to know to setFlush on an ObjectBox preceding a timeline, we can make that automatic with CSS.

Test Plan: Test some Diffusion commit pages, boxes now site flush under timeline. Check Maniphest and Differential, no changes.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11617