Maintenance form fields are not compliant with Public Code standard

[valeriocomo]

Subject of the issue

Form fields maintenance.contracts and maintenance.contractors don’t respect proper dependency from the maintenance.type property value, as reported in #349, issue 5.

Steps to reproduce

Select maintenance.type and you should see almost one between maintenance.contracts and maintenance.contractors form field.

Expected behaviour

Should be shown almost one between maintenance.contracts and maintenance.contractors form field according to the standard.

Actual behaviour

maintenance.contracts and maintenance.contractors are both shown regardless of the value of maintenance.type