[Mantis] enable due date

• if you use google cloud deployment to deploy bitnami mantis service
• use ssh to login shell
• edit /opt/bitnami/apps/mantis/htdocs/config/config_inc.php
• add following lines in config/config_inc.php

$g_due_date_update_threshold = DEVELOPER;
$g_due_date_view_threshold = REPORTER;

• Reference:
• adding due date field
• Bitnami doc