Add a message for the case of files which can not be deleted because they...

Add a message for the case of files which can not be deleted because they where referenced by others.

A player referenced in a unit can not be deleted without deleting the unit. A Unit used in a booklet can not be deleted without deleting the booklet. A booklet used in a
Testtakers-File can not be deleted without that.

src/app/workspace-admin/backend.service.ts

@@ -179,4 +179,5 @@ export interface FileDeletionReport {
   deleted: string[];
   not_allowed: string[];
   did_not_exist: string[];
+  was_used: string[];
 }

src/app/workspace-admin/files/files.component.ts

@@ -131,6 +131,10 @@ export class FilesComponent implements OnInit {
             if (fileDeletionReport.not_allowed.length > 0) {
               message.push(`${fileDeletionReport.not_allowed.length} Dateien konnten nicht gelöscht werden.`);
             }
+            if (fileDeletionReport.was_used.length > 0) {
+              message.push(`${fileDeletionReport.was_used.length} Dateien werden von anderen verwendet 
+              und wurden nicht gelöscht.`);
+            }
             this.snackBar.open(message.join('<br>'), message.length > 1 ? 'Achtung' : '', { duration: 1000 });
             this.updateFileList();
           });