Make home folders movable without requiring rescans

This introduces a new `Home` storage backend which provides access to the users home directory without the storage id changing whenever the home folder changes for the user.

If no existing `Local` storage exists for the user's home this new backend is used

This makes it possible for the users home folder to be dynamic without forcing rescans while still being backwards compatible

cc @karlitschek @DeepDiver1975 @bantu