Fix 404 clicking Find Owners in diffusion, if Owners application is disabled.

Test Plan: Install/uninstall Owners application, Find Owners action shown/hidden as expected.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7557