react-hook-form button is not disabled for long enough

### What do you want and why?
I am using react-hook-form and the `new.tsx` pages that get generated always have this part:
``

This means that after the form has been submitted successfully and `isSubmitting` is false again, we can double submit the form before the route changes (especially on slower connections) and create a second new entity by accident.

I'd like to do something like `disabled={ctx.formState.isSubmitSuccessful || ctx.formState.isSubmitting}` to prevent this accidental double submit.

### Possible implementation(s)
Adding `ctx.formState.isSubmitSuccessful` should be possible pretty easy as documented in [react-hook-form formState documentation](https://react-hook-form.com/api/useform/formstate).