We need to be able to edit a few parts of the voucher after it's been generated. The fields that should be editable are:
Attempting to edit any other fields should result in an appropriate error.
NOTE: protected fields logic can be borrowed from mod-orders
- API is defined and implemented (RAML/ModDesc/Code)
- Validation of which fields are edited is performed
- Appropriate error message are used when there are attempts to modify fields which are "off limits"
- Unit tests are updated
- API tests are updated