refactoring: extract common form_valid() method
refactoring: extract common form_valid() method

of OrderNewExcerptView and OrderExistingExcerptView

geometalab/osmaxx,geometalab/osmaxx,geometalab/osmaxx,geometalab/osmaxx-frontend,geometalab/drf-utm-zone-info,geometalab/drf-utm-zone-info,geometalab/osmaxx-frontend,geometalab/osmaxx-frontend,geometalab/osmaxx-frontend,geometalab/osmaxx