PloneSite referencenumber: use current admin_units abbreviation instead of the label.

With the introduction of admin and org units the referencenumber adapter for `PloneSites` has changed, to returning the current admin_units label. But this approach is to less flexible, therefore we included a new field abbreviation (see https://github.com/4teamwork/opengever.ogds.models/pull/24) which is used in the PloneSite's referencenumber adapter.

PR Includes an upgradestep for the new column.

@deiferni please have a look.