Allow setting of tabs at StandardPageView

Summary: Rather than have tabs live in two column view, sometimes like admin we'll want a global set of tabs that work well with all layouts and crumbs.

Test Plan:
I tested this in an upcoming diff for instances.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18332