[ACA-1591] Load extensions from multiple files (#521)

* rework extension service, separate file with config

* improve loading, optional entries

* simplify config and unify content actions

* load and merge multiple files

* improve plugin loading, introduce second demo

* move demo stuff to a plugin

* rework navbar to make it pluggable

* code and naming convention cleanup

* extension schema

* switch off custom navbar group by default

* hotfix for facetQueries issue

* consolidate files, final renames

src/app/app.component.ts

@@ -91,8 +91,6 @@ export class AppComponent implements OnInit {
                 pageTitle.setTitle(data.title || '');
             });
 
-        this.extensions.init();
-
         this.router.config.unshift(...this.extensions.getApplicationRoutes());
 
         this.uploadService.fileUploadError.subscribe(error =>