readonly_fields for ModelForms

[Lacrymology]

I think it'd be very useful for ModelForm to support the same behavior as ModelAdmin's readonly_fields.

If not, here's a plausible ReadonlyWidget code.

class ReadOnlyWidget(forms.Widget):

    def __init__(self, original_value, display_value):
        self.original_value = original_value
        self.display_value = display_value
        super(ReadOnlyWidget, self).__init__()

    def _has_changed(self, initial, data):
        return False

    def render(self, name, value, attrs=None):
        if self.display_value is not None:
            return unicode(self.display_value)
        return unicode(self.original_value)

    def value_from_datadict(self, data, files, name):
        return self.original_value

[Aymeric Augustin]

Duplicate of #17031.