[prep-release-3.1.0-RC5] Update version to 3.1.0-RC5
[prep-release-3.1.0-RC5] Add migration for 3.1.0-RC5
[prep-release-3.1.0-RC5] Prepare Changelog
[prep-release-3.1.0-RC5] Add changelog for 3.1.0-RC5
[prep-release-3.1.0-RC5] Update phpBB Version for RC6 development
Merge branch 'prep-release-3.1.0-RC5' into develop-ascraeus