[prep-release-3.1.0-RC3] Add migration for 3.1.0-RC3
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [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

Conflicts:
	phpBB/styles/subsilver2/style.cfg