Dashboards - add remove functionality

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