[prep-release-3.1.0-RC4] Update version to 3.1.0-RC4
[prep-release-3.1.0-RC4] Add migration for 3.1.0-RC4
[prep-release-3.1.0-RC4] Prepare Changelog
[prep-release-3.1.0-RC4] Add changelog for 3.1.0-RC4
[prep-release-3.1.0-RC4] Update phpBB Version for RC4 development
Merge branch 'prep-release-3.1.0-RC4' into develop-ascraeus

* prep-release-3.1.0-RC4:
  [prep-release-3.1.0-RC4] Update phpBB Version for RC4 development
  [prep-release-3.1.0-RC4] Add changelog for 3.1.0-RC4
  [prep-release-3.1.0-RC4] Prepare Changelog
  [prep-release-3.1.0-RC4] Add migration for 3.1.0-RC4
  [prep-release-3.1.0-RC4] Update version to 3.1.0-RC4