Added the possibility to convert a subtask to a task

app/Controller/SubtaskConverterController.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace Kanboard\Controller;
+
+/**
+ * Class SubtaskConverterController
+ *
+ * @package Kanboard\Controller
+ * @author  Frederic Guillot
+ */
+class SubtaskConverterController extends BaseController
+{
+    public function show()
+    {
+        $task = $this->getTask();
+        $subtask = $this->getSubtask();
+
+        $this->response->html($this->template->render('subtask_converter/show', array(
+            'subtask' => $subtask,
+            'task' => $task,
+        )));
+    }
+
+    public function save()
+    {
+        $project = $this->getProject();
+        $subtask = $this->getSubtask();
+
+        $task_id = $this->subtask->convertToTask($project['id'], $subtask['id']);
+
+        if ($task_id !== false) {
+            $this->flash->success(t('Subtask converted to task successfully.'));
+        } else {
+            $this->flash->failure(t('Unable to convert the subtask.'));
+        }
+
+        $this->response->redirect($this->helper->url->to('task', 'show', array('project_id' => $project['id'], 'task_id' => $task_id)), true);
+    }
+}