AdminUnit table: Added upgradestep for the new abbreviation column.

See https://github.com/4teamwork/opengever.ogds.models/pull/24 for more informations.
Referencenumber: use current admin_units abbreviation instead of the label for the PloneSite part.
PEP8.
Merge pull request #600 from 4teamwork/pg_abbreviation_column

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