R1:95925ad58ffb

Make getApplicationTransactionEditor() in PhabricatorApplication return an editor

Summary:
Ref T12685. I provided this incorrect (`return new` rather than `throw`) implementation earlier; it can now be replaced with a proper implementation.

This caused application policy edits to spew this into the daemon log:

```
[2017-05-14 15:35:27] EXCEPTION: (Error) Call to undefined method PhutilMethodNotImplementedException::setActor() at…