Allow dashboard panels to render in a headerless mode

Summary:
Ref T4986. This mostly just makes tab panels a little nicer.

Maybe this will be modal (header = "none", "edit", "view") in a few diffs but we can clean it up then if so.

Test Plan:

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4986

Differential Revision: https://secure.phabricator.com/D9143