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