Add edit/view plumbing for dashboards and panels

Summary: Ref T3583. This doesn't add any dashboard/panel-specific code beyond headers/titles/buttons/etc., but allows you to create and view dashboards and panel skeletons.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3583

Differential Revision: https://secure.phabricator.com/D8131