[prep-release-3.1.0-RC3] Update version to 3.1.0-RC3
[prep-release-3.1.0-RC3] Add migration for 3.1.0-RC3
[prep-release-3.1.0-RC3] Prepare Changelog
[prep-release-3.1.0-RC3] Add changelog for 3.1.0-RC3
[prep-release-3.1.0-RC3] Update phpBB Version for RC4 development
Merge branch 'prep-release-3.1.0-RC3' into develop-ascraeus

* prep-release-3.1.0-RC3:
  [prep-release-3.1.0-RC3] Update phpBB Version for RC4 development
  [prep-release-3.1.0-RC3] Add changelog for 3.1.0-RC3
  [prep-release-3.1.0-RC3] Prepare Changelog
  [prep-release-3.1.0-RC3] Add migration for 3.1.0-RC3
  [prep-release-3.1.0-RC3] Update version to 3.1.0-RC3