When cancelling addition of an Almanac interface, return to the Device page

Summary:
Fixes T13184. In Almanac, interfaces are always added to devices. However, if you "Add New Interface" and then "Cancel", you go to the nonexistent `/interface/` page.

Instead, return to the device page.

Test Plan: From a device page, clicked "Add Interface" and then "Cancel". Ended up back where I was.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13184

Differential Revision: https://secure.phabricator.com/D19573

src/applications/almanac/editor/AlmanacInterfaceEditEngine.php

@@ -127,6 +127,9 @@ final class AlmanacInterfaceEditEngine
   }
 
   protected function getObjectCreateCancelURI($object) {
+    if ($this->getDevice()) {
+      return $this->getDevice()->getURI();
+    }
     return '/almanac/interface/';
   }