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

app/Core/Base.php

@@ -76,6 +76,7 @@ use Pimple\Container;
  * @property \Kanboard\Formatter\TaskGanttFormatter                  $taskGanttFormatter
  * @property \Kanboard\Formatter\TaskICalFormatter                   $taskICalFormatter
  * @property \Kanboard\Formatter\TaskListFormatter                   $taskListFormatter
+ * @property \Kanboard\Formatter\TaskListSubtaskFormatter            $taskListSubtaskFormatter
  * @property \Kanboard\Formatter\TaskSuggestMenuFormatter            $taskSuggestMenuFormatter
  * @property \Kanboard\Formatter\UserAutoCompleteFormatter           $userAutoCompleteFormatter
  * @property \Kanboard\Formatter\UserMentionFormatter                $userMentionFormatter

app/Core/Session/SessionStorage.php

@@ -18,7 +18,7 @@ namespace Kanboard\Core\Session;
  * @property string $commentSorting
  * @property bool   $hasSubtaskInProgress
  * @property bool   $hasRememberMe
- * @property bool   $boardCollapsed
+ * @property bool   $subtaskListToggle
  * @property string $scope
  * @property bool   $twoFactorBeforeCodeCalled
  * @property string $twoFactorSecret

app/Core/User/UserSession.php

@@ -145,6 +145,17 @@ class UserSession extends Base
         return isset($this->sessionStorage->user['username']) ? $this->sessionStorage->user['username'] : '';
     }
 
+    /**
+     * Return true if subtask list toggle is active
+     *
+     * @access public
+     * @return string
+     */
+    public function hasSubtaskListActivated()
+    {
+        return isset($this->sessionStorage->subtaskListToggle) && ! empty($this->sessionStorage->subtaskListToggle);
+    }
+
     /**
      * Check is the user is connected
      *