[prep-release-3.1.0-b3] Update version to 3.1.0-b3
[prep-release-3.1.0-b3] Add migration for 3.1.0-b3
[prep-release-3.1.0-b3] Prepare Changelog
[prep-release-3.1.0-b3] Add changelog for 3.1.0-b3
[prep-release-3.1.0-b3] Update phpBB Version for b4 development
Merge branch 'prep-release-3.1.0-b3' into develop-ascraeus

* prep-release-3.1.0-b3:
  [prep-release-3.1.0-b3] Update phpBB Version for b4 development
  [prep-release-3.1.0-b3] Add changelog for 3.1.0-b3
  [prep-release-3.1.0-b3] Prepare Changelog
  [prep-release-3.1.0-b3] Add migration for 3.1.0-b3
  [prep-release-3.1.0-b3] Update version to 3.1.0-b3