Move subtask forecast to a plugin

Plugin repo: https://github.com/kanboard/plugin-subtask-forecast

app/Controller/Config.php

@@ -48,7 +48,7 @@ class Config extends Base
                     $values += array('integration_slack_webhook' => 0, 'integration_hipchat' => 0, 'integration_gravatar' => 0, 'integration_jabber' => 0);
                     break;
                 case 'calendar':
-                    $values += array('calendar_user_subtasks_forecast' => 0, 'calendar_user_subtasks_time_tracking' => 0);
+                    $values += array('calendar_user_subtasks_time_tracking' => 0);
                     break;
             }