Change "Ansprechperson E-Mail" into free-form "Ansprechperson Kontakt"

The following discussion from !122 (merged) should be addressed by replacing the mail-typed "Ansprechperson E-Mail" by a free-form "Ansprechperson Kontakt" field, migrating the existing data into the new field, dropping the old one and adjusting the bulk import script accordingly. (This will need two deployments, one to add the new field and migrate the data into it and another to delete the old field after the migration is done.)