[Redesign] Make ObjectBox Header conditional

Summary: Ref T8099. In cases like dashboards, we don't always want a header with ObjectBox. This makes it optional.

Test Plan: Review a tab panel, see no weird lines.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D12920