Make passing an object to newCurtain optional

Summary: We seem to already support this, just takes it fully there. We don't need to see things like "Flag", etc, on certain subpages of projects/people/etc.

Test Plan: Review Members, Subproject pages, no longer see "Flag for Later" which only is for the Project itself. Check manage, still there.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17897