Use panels for spacing in Maniphest.

Summary: Trying to move move content areas to panelview for consistency in spacing.

Test Plan: Reload Maniphest pages, see equal spacing like on Differential.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4527