Quota storage wrapper is now used for all users in sharing mode

When accessing a shared folder, the folder's owner appears as mountpoint
but wasn't wrapped by a quota storage wrapper.

This fix makes sure that all home storages are wrapped by a quota
storage wrapper, if applicable, to make sure quotas are respected when
uploading into shared folders.

@icewind1991 are "local://" based home dirs migrated to "home://" storages or should I add extra handling for those ?

Please review @icewind1991 @DeepDiver1975 @karlitschek 

I'll add unit tests soon.