backport home storage to stable5

backport of https://github.com/owncloud/core/pull/5591 for stable5 necessary for user backends that implement getHome(). fixes https://github.com/owncloud/core/issues/5045

@icewind1991 I tried to backport this because I need it for a customer migration. please review

@karlitschek @DeepDiver1975 @VicDeo please review