FIX EZP-23121: assignRole allows duplicate role/limitation assignments

New approach for #926, issue : https://jira.ez.no/browse/EZP-23121

Description:
Role Service assignRoleToUser/Group allows creating duplicate role/limitation assignments.
This can be verified in legacy admin (after clearing cache) or in database.

Todo:
- [x] adjust unit tests
- [x] add some more integration coverage (suggestions welcome) 
Ok, it seems I'm in the wrong here, considering we already use the same in similar cases, e.g. https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/API/Repository/ContentTypeService.php#L233.
Ignore it then, +1 stands :)