R1:905fc217b80c

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…