202 Geometry conflict checking to background

* wraps check_for_conflicts() in celery based method
* fixes bug on InterventionEditForm where geometry's save() has been called twice

intervention/forms/intervention.py

@@ -372,7 +372,6 @@ class EditInterventionForm(NewInterventionForm):
 
             geometry = geom_form.save(user_action)
             self.instance.geometry = geometry
-            self.instance.geometry.save()
 
             self.instance.log.add(user_action)