UserDao.removeAttribute does not keep InternalUser.attributes property up to date

With two way mappings of data in Hibernate, you need to add extra code to ensure that both directions of the mappings are kept up to date. Failing to do so can lead to hibernate attempting to re-save a removed object in the mapping collection, or other bad side effects.

In this case, that means that whenever an attribute is added, updated or removed, its entry in the InternalUser.attributes set also needs to be added, updated or removed. The same goes for InternalGroup.attributes.