Update form handling for changes in hotwired/turbo 7.0.0-beta.2:
- **TurboStreamFormMixin** class and supporting classes removed
- **TurboFormMixin** class added that just returns a 422 response on invalid
- **TurboStreamFormView**, **TurboStreamCreateView** and **TurboStreamUpdateView** classes removed
- **TurboFormView**, **TurboCreateView** and **TurboUpdateView** classes added, using new **TurboFormMixin**