[prep-release-3.1.0-RC2] Update version to 3.1.0-RC2
[prep-release-3.1.0-RC2] Add migration for 3.1.0-RC2
[prep-release-3.1.0-RC2] Prepare Changelog
[prep-release-3.1.0-RC2] Add changelog for 3.1.0-RC2
[prep-release-3.1.0-RC2] Update phpBB Version for RC3 development
Merge branch 'prep-release-3.1.0-RC2' into develop-ascraeus

* prep-release-3.1.0-RC2:
  [prep-release-3.1.0-RC2] Update phpBB Version for RC3 development
  [prep-release-3.1.0-RC2] Add changelog for 3.1.0-RC2
  [prep-release-3.1.0-RC2] Prepare Changelog
  [prep-release-3.1.0-RC2] Add migration for 3.1.0-RC2
  [prep-release-3.1.0-RC2] Update version to 3.1.0-RC2