Added upgradestep to harmonize column lengths of groupid and userid columns

Right now the `user` resp. the `group` table defines an other column size than the foreign_key tables does.

This PR solves this issue. Schema changes in the `opengever.ogds.models` package (see https://github.com/4teamwork/opengever.ogds.models/pull/25)

@deiferni please have a look ...