DOI ID auf die erste Seite #16

- query additional 'identifier' field in solr query: dataservice.ts
- if document identifier is available, show it on the search page: vs-results.vue

resources/js/search/search-results/dataservice.ts

@@ -27,6 +27,7 @@ export default {
     var fields = ["id", "licence",
       "server_date_published",
       "abstract_output",
+      "identifier",
       "title_output",
       "title_additional",
       "author",