Use Ajax request for Markdown preview

This remove dependency on a Javascript library.

app/Console/JsCommand.php

@@ -66,8 +66,6 @@ class JsCommand extends BaseCommand
         'assets/vendor/d3/d3.min.js',
         'assets/vendor/c3/c3.min.js',
         'assets/vendor/isMobile/isMobile.min.js',
-        'assets/vendor/marked/marked.min.js',
-        'assets/vendor/dompurify/purify.min.js',
     ];
 
     protected function configure()

app/Controller/TaskAjaxController.php

@@ -9,6 +9,7 @@ use Kanboard\Filter\TaskStartsWithIdFilter;
 use Kanboard\Filter\TaskStatusFilter;
 use Kanboard\Filter\TaskTitleFilter;
 use Kanboard\Model\TaskModel;
+use Kanboard\Helper\TextHelper;
 
 /**
  * Task Ajax Controller
@@ -67,4 +68,21 @@ class TaskAjaxController extends BaseController
             $this->response->json($filter->format($this->taskSuggestMenuFormatter));
         }
     }
+
+    /**
+     * Task edit preview
+     */
+    public function preview()
+    {
+        $text = $this->request->getRawValue('text');
+
+        if (empty($text)) {
+            $this->response->json(array());
+        } else {
+            $textHelper = new TextHelper($this->container);
+            $preview = $textHelper->markdown($text);
+            $this->response->json(array($preview));
+        }
+    }
+
 }

app/Helper/FormHelper.php

@@ -233,6 +233,7 @@ class FormHelper extends Base
             'required' => isset($attributes['required']) && $attributes['required'],
             'tabindex' => isset($attributes['tabindex']) ? $attributes['tabindex'] : '-1',
             'labelPreview' => t('Preview'),
+            'previewUrl' => $this->helper->url->to('TaskAjaxController', 'preview'),
             'labelWrite' => t('Write'),
             'labelTitle' => t('Title'),
             'placeholder' => t('Write your text in Markdown'),