Add complete task functionality (#71)

closes #61
closes #58

Reviewed-on: #71

frontend/allowance-planner-v2/src/app/services/task.service.ts

@@ -26,4 +26,8 @@ export class TaskService {
     updateTask(id: number, task: Partial<Task>) {
         this.http.put(`${this.url}/task/${id}`, task).subscribe();
     }
+
+    completeTask(id: number) {
+        this.http.post(`${this.url}/task/${id}/complete`, {}).subscribe();
+    }
 }