Set a URI on Auth Messages, so the "Change Details" dialog from the transaction record has a cancel button

Summary:
If you edit an auth message in Auth > Customize Messages, then click "Show Details" in the transaction record, the resulting dialog uses the object's handle's URI to generate a "cancel" button.

Since these handles currently have no URI, the dialog currently has no cancel/done button to close it.

Test Plan: Edited an auth message, clicked "Show Details", was now able to click "Done" to close the dialog.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20471