Fix firefox submenu arrows on action lists

Summary: Fixes T12370. Moves this to be absolutely positions so the float doesn't mess up text-overflow layouts.

Test Plan: Chrome, Safari, Firefox, mobile and desktop layouts of Maniphest submenus.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12370

Differential Revision: https://secure.phabricator.com/D17484