Phriction - if you can't edit x/y don't allow creating x/y/z

Summary: ...how do you lock down entire areas otherwise? Fixes T6496.

Test Plan: used user 1 to create x/y that user 2 can't edit. tried to create x/y/z as user 2 and got a big ole error dialogue.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6496

Differential Revision: https://secure.phabricator.com/D10819