(front) Service refactoring for sorting operations

- Create an ElsArtistService to use common query
- Move services in a subfolder

Use a function to add sort filter

dashboard/src/app/els-service/els-album.service.spec.ts

@@ -0,0 +1,16 @@
+import { TestBed } from '@angular/core/testing';
+
+import { ElsAlbumService } from './els-album.service';
+
+describe('ElsAlbumService', () => {
+  let service: ElsAlbumService;
+
+  beforeEach(() => {
+    TestBed.configureTestingModule({});
+    service = TestBed.inject(ElsAlbumService);
+  });
+
+  it('should be created', () => {
+    expect(service).toBeTruthy();
+  });
+});