Preventing users from buying already gifted subscriber items (#7734)

* Adding the unopened mystery items to the call to get not obtained subscriber items. closes #7712

* refactoring according to pr

* Refactoring according to pr. moved time-travelers to it's own file and added new tests.

test/client-old/spec/controllers/inventoryCtrlSpec.js

@@ -28,7 +28,10 @@ describe('Inventory Controller', function() {
         },
         preferences: {
           suppressModals: {}
-        }
+        },
+        purchased: {
+          plan: {}
+        },
       });
 
       Shared.wrap(user);