Fix URL generation for relationships nested API

[glennmatthews]

Closes: DNE

What's Changed

Fix URL generation for nested relationships data in the REST API so that it includes the full path like other url fields.

Before:

            "relationships": {
                "sites-vrfs": {
                    "id": "6736bd25-27cf-4200-a0cf-054eeeb10555",
                    "url": "/api/extras/relationships/6736bd25-27cf-4200-a0cf-054eeeb10555/",   <----<<
                    "name": "Site's VRFs",
                    "type": "many-to-many",
                    "destination": {
                        "label": "Related VRFs",
                        "object_type": "ipam.vrf",
                        "objects": [
                            {
                                "id": "9641b4af-6ab4-4c99-9df2-1e019ed3e546",
                                "url": "http://localhost:8080/api/ipam/vrfs/9641b4af-6ab4-4c99-9df2-1e019ed3e546/",
                                "name": "Red",
                                "rd": null,
                                "display": "Red"
                            }
                        ]
                    }
                }
            }

After:

            "relationships": {
                "sites-vrfs": {
                    "id": "6736bd25-27cf-4200-a0cf-054eeeb10555",
                    "url": "http://localhost:8080/api/extras/relationships/6736bd25-27cf-4200-a0cf-054eeeb10555/",  <------<<
                    "name": "Site's VRFs",
                    "type": "many-to-many",
                    "destination": {
                        "label": "Related VRFs",
                        "object_type": "ipam.vrf",
                        "objects": [
                            {
                                "id": "9641b4af-6ab4-4c99-9df2-1e019ed3e546",
                                "url": "http://localhost:8080/api/ipam/vrfs/9641b4af-6ab4-4c99-9df2-1e019ed3e546/",
                                "name": "Red",
                                "rd": null,
                                "display": "Red"
                            }
                        ]
                    }
                }
            }