MFH: r494382

Update to 1.8.20

Changelog: https://blog.mybb.com/2019/02/27/mybb-1-8-20-released-security-maintenance-release/

Security: 395ed9d5-3cca-11e9-9ba0-4c72b94353b5
Sponsored by: Netzkommune GmbH

Approved by: ports-secteam (joneum)