(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/album/album.component.html

@@ -1,7 +1,7 @@
 <div class="container">
   <h1>{{albumName}}</h1>
 
-  <div *ngIf="toSort" class="alert alert-danger">
+  <div *ngIf="sortFilter" class="alert alert-danger">
     To sort filter applyed. Songs are filter on location. <br/>
     Album folder: {{album['Location']}}
   </div>