Add toggle button to show/hide subtasks in task list view

app/Formatter/TaskListSubtaskFormatter.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Kanboard\Formatter;
+
+/**
+ * Class TaskListSubtaskFormatter
+ *
+ * @package Kanboard\Formatter
+ * @author  Frederic Guillot
+ */
+class TaskListSubtaskFormatter extends TaskListFormatter
+{
+    /**
+     * Apply formatter
+     *
+     * @access public
+     * @return array
+     */
+    public function format()
+    {
+        $tasks = parent::format();
+        $taskIds = array_column($tasks, 'id');
+        $subtasks = $this->subtaskModel->getAllByTaskIds($taskIds);
+        $subtasks = array_column_index($subtasks, 'task_id');
+        array_merge_relation($tasks, $subtasks, 'subtasks', 'id');
+
+        return $tasks;
+    }
+}