Replace monkey patching on deform classes by templating

[arnaud-morvan]

We should replace https://github.com/camptocamp/c2cgeoform/blob/996930b7943144384449a059525a6df1c9fe6f2c/c2cgeoform/__init__.py#L88:L89

By changes in templates form.pt and mapping.pt:

<form
  tal:define="
    fields_template fields_template|string:${field.widget.fields_template or field.fields_template or 'mapping_template'};