[mob] Set old prop while creating shared link

mobile/lib/services/collections_service.dart

@@ -950,6 +950,8 @@ class CollectionsService {
           "collectionID": collection.id,
           "enableCollect": enableCollect,
           "enableJoin": flagService.internalUser,
+          "joinViaLink":
+              flagService.internalUser, // todo: remove after server deployment
         },
       );
       collection.publicURLs?.add(PublicURL.fromMap(response.data["result"]));