[prep-release-3.1.0-RC1] Update version to 3.1.0-RC1
[prep-release-3.1.0-RC1] Add migration for 3.1.0-RC1
[prep-release-3.1.0-RC1] Prepare Changelog
[prep-release-3.1.0-RC1] Add changelog for 3.1.0-RC1
[prep-release-3.1.0-RC1] Update phpBB Version for RC2 development
Merge branch 'prep-release-3.1.0-RC1' into develop-ascraeus

* prep-release-3.1.0-RC1:
  [prep-release-3.1.0-RC1] Update phpBB Version for RC2 development
  [prep-release-3.1.0-RC1] Add changelog for 3.1.0-RC1
  [prep-release-3.1.0-RC1] Prepare Changelog
  [prep-release-3.1.0-RC1] Add migration for 3.1.0-RC1
  [prep-release-3.1.0-RC1] Update version to 3.1.0-RC1