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 [<phabricator>/src/applications/transactions/worker/PhabricatorApplicationTransactionPublishWorker.php:69]
```
Test Plan:
- Used `bin/worker execute --id <id>` to execute a previously-failing task.
- Saw a feed story publish.
Reviewers: chad, amckinley
Reviewed By: chad
Maniphest Tasks: T12685
Differential Revision: https://secure.phabricator.com/D17876