Summary: To get there, upgrade "headerless" to "headerMode". Add a new removepanel controller. Fixes T5084.
Test Plan: removed some panels to much success
Reviewers: chad, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T5078, T5084
Differential Revision: https://secure.phabricator.com/D9156
