[prep-release-3.1.0] Add missing CHANGELOG entries for RC6
Merge branch 'prep-release-3.1.0' into develop-ascraeus

* prep-release-3.1.0:
  [prep-release-3.1.0] Add missing CHANGELOG entries for RC6
[ticket/13207] Use assert_checkbox_is_checked for registration test

PHPBB3-13207