[prep-release-3.1.0-b4] Update version to 3.1.0-b4
[prep-release-3.1.0-b4] Add migration for 3.1.0-b4
[prep-release-3.1.0-b4] Prepare Changelog
[prep-release-3.1.0-b4] Add changelog for 3.1.0-b4
[prep-release-3.1.0-b4] Update phpBB Version for b5 development
Merge branch 'prep-release-3.1.0-b4' into develop-ascraeus

* prep-release-3.1.0-b4:
  [prep-release-3.1.0-b4] Update phpBB Version for b5 development
  [prep-release-3.1.0-b4] Add changelog for 3.1.0-b4
  [prep-release-3.1.0-b4] Prepare Changelog
  [prep-release-3.1.0-b4] Add migration for 3.1.0-b4
  [prep-release-3.1.0-b4] Update version to 3.1.0-b4