Replace form_for with form_with

[goosys]

Since form_for has been deprecated in Rails 5.1, I replaced it with form_with.
The change required minimal adjustments thanks to good compatibility.

In the demo app, we need to set config.action_view.form_with_generates_ids = true to automatically generate id and for attributes.
However, this is already true by default in load_defaults from Rails 6.0 onward, so it shouldn’t cause problems in most applications.

[nickcharlton]

Thank you!