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