Oversight: Prevent deletion/movement of stub pages

Summary: Refs T2686 - Now you also can't delete / move stubbed pages

Test Plan: tried it out - works wonderful

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2686

Differential Revision: https://secure.phabricator.com/D5341