implemented adding sub tasks

frontend/src/routes/task/View.svelte

@@ -21,6 +21,10 @@
         loadTask();
     }
 
+    function addChild(){
+        router.navigate(`/task/${id}/add_subtask`);
+    }
+
     async function loadChildren(){
         const url = api('task/list');
         var data = {
@@ -127,6 +131,7 @@
         <tr>
             <th>{t('subtasks')}</th>
             <td class="task children">
+                <button onclick={addChild} >{t('add_subtask')}</button>
                 <TaskList tasks={children} {estimated_time} />
             </td>
         </tr>