Fix Calendar/Double Panel Headers

Summary: Fixes T8829. Moves the padding and background to the outer container instead of the table itself.

Test Plan: Test Calendar Month View, Calendar Day View, A Text Panel with Multiple Dashboards.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8829

Differential Revision: https://secure.phabricator.com/D13631