Add a dedicated permission to edit the field_billwerk_contract_id field in user profiles

Problem/Motivation

Add a dedicated permission to edit the field_billwerk_contract_id field in user profiles. So administrators with permission are allowed to edit the field (#access = TRUE) while other users are not and we don't need field_permissions module for that.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes