CHANGE: Prevent both person and org being left blank for a Rig

Of course, there's no requirement that either of those have any actual contact details...

Closes #276.

RIGS/forms.py

@@ -129,6 +129,11 @@ class EventForm(forms.ModelForm):
 
         return item
 
+    def clean(self):
+        if self.cleaned_data.get("is_rig") and not (self.cleaned_data.get('person') or self.cleaned_data.get('organisation')):
+            raise forms.ValidationError('You haven\'t provided any client contact details. Please add a person or organisation.', code='contact')
+        return super(EventForm, self).clean()
+
     def save(self, commit=True):
         m = super(EventForm, self).save(commit=False)