make sure user attributes are always sorted properly

when you view a user the attributes are sorted properly, but when you switch into the edit mode, the order is different.

There's an ORDER BY in getList (http://api.concrete5.ch/source-class-Concrete5_Model_AttributeKey.html#125), but that doesn't work in this case because the display order is category specific and thus only found in `UserAttributeKeys`, this PR should fix that.