Issue-#1311:Permissions summary does not properly display the name of gr...

...oups
There's no need to call `ownerReference.toString()`, for several reasons:

- Velocity automatically converts parameters to strings, if a method with the requested name and provided parameter type doesn't exist but a method with `String` parameters exists
- using strings as document names is deprecated, you should try to use `DocumentReference`s as often as possible
- while `Owner.getUser()` officially returns an `EntityReference`, the actual type is `DocumentReference`, and there is an `XWiki.getDocument(DocumentReference)` method, which is the best one to use