add redirection ability for a successful form submission to show a customized confirmation page

It would be great if we can show up a confirmation with customized message page after a successful submission.
Please refer to https://www.boxvoyageworkbench.com/en/contact-us/ or https://www.boxvoyageworkbench.com/en/products/schedule-reliability-application/download/

after successful submission page will goes to https://www.boxvoyageworkbench.com/en/contact-us/confirmation/, 
https://www.boxvoyageworkbench.com/en/products/schedule-reliability-application/download-confirmation/

They are all defined in a newly added field in `forms_builder.forms.model.AbstractForm` called `redirect_url`.

People would need to run below command for this feature's adoption.
```python
python manage.py migrate forms_builder.forms 0007
```