Django-frontend-forms

- [fix] "it" language selection for datepicker
- formdata_serialize() and formdata_to_querystring() helpers added

* diplay_message() helper added

* small style fix for checkbox in horizontal generic form rendering

* revised example project

* Prepare for Django 4.0

* POSSIBLE INCOMPATIBLE CHANGE: Radio button layout refactored in "render_form_field.html"
* send "submission_failure" notification
* example refactoring wip ...
* package up data from form with FormData (instead of form.serialize()) to allow files upload
* allow customization of `enctype` in generic form template
* send "submission_failure" notification