accounting: entitymodel: fix Datev* entity relation fk-names
accounting: entitymodel: fix Datev* entity relation fk-names

These were too long, should be max 18 chars.

Refs https://gitlab.ilscipio.com/scipio-dev/dev/scipioce-dev/issues/619
ilscipio/scipio-erp,ilscipio/scipio-erp,ilscipio/scipio-erp,ilscipio/scipio-erp,ilscipio/scipio-erp