Move timeline anchor targets up to evade a Chrome 55 behavioral change

Summary: Fixes T11997. This lifts the targets out of the containing overflow: hidden; div so Chrome is willing to target them.

Test Plan: In Chrome, visited direct comment links and ended up in the right place. Clicked comment anchors, saw browser jump around again.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11997

Differential Revision: https://secure.phabricator.com/D17033