MFH: r504029

Update to 1.8.21

Changelog: https://blog.mybb.com/2019/06/10/mybb-1-8-21-released-security-maintenance-release/

Security: 13960f55-8d35-11e9-9ba0-4c72b94353b5
Sponsored by: Netzkommune GmbH

Approved by: ports-secteam (joneum)