jquery-ujs automatically disable submit button when submitting the form.
If one choose cancel on the leaving page warning, then the submit buttons
end up in a disable state, with no way to re enable them. This fix
prevent the warning from being triggered when submitting the form, so
we can't end up in the scenario described.
Buttons will be enabled once the form has been interacted with.
Update unsavedChanges stimulus controller to handle this. It should
still be generic enought that it can be reused.
Add UnsavedChanges stimulus controller, it should be generic enough so
that it can reused somewhere else. It works with both 'beforeunload' event
and 'turbolinks:before-visit' when using turbo links.
