Phriction - validateTransactions that need parent ancestry to complete successfully

Summary:
Fixes T6651, T6682. Since policy is defined by ancestry, you can't make things outside the core tree.

An alternative fix would be to automagically stub everything in these cases. This has potential negative policy implications - consider making a public document with several levels of depth that automagically stubs out its ancestry as public - and additionally the PhabricatorApplicationTransactionEditor framework would make this very tricky code (i.e. you are expected to validateTransactions in said hook *and* return an error if things aren't valid and not do some automagic stubbing, etc.)

Test Plan: tried to move a doc from location/that/exists to locationz/thatz/dontz/existz/ and got an error message with links to each missing doc. tried to create a doc at locatonz/thatz/dontz/existsz/ and got an error message with links to each missing doc.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6682, T6651

Differential Revision: https://secure.phabricator.com/D10978