MFH: r479518
www/mybb: Update to 1.8.18
Changelog: https://blog.mybb.com/2018/08/22/mybb-1-8-18-released-security-maintenance-release/
Security: db2acdac-b5a7-11e8-8f6f-00e04c1ea73d
Sponsored by: Netzkommune GmbH
Approved by: ports-secteam (miwi)