Fix an error in Diffusion when the Owners application is uninstalled

Summary:
See https://discourse.phabricator-community.org/t/undefined-view-when-owners-is-uninstalled/451.

When Owners is not installed, Diffusion can fatal with a bad $view.

Test Plan:

• Uninstall Owners.
• View the content of any file in Diffusion.
• Before: fatal on $view undefined.
• After: Valid page with no owners information.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18629