Do not require chamber of commerce number in signup form

[WKobes]

Currently we require KvK number: https://github.com/internetstandards/Internet.nl-dashboard-frontend/blob/282b73573b2dc0d2c47f61742346379eb6c88731/src/components/signup/SignupForm.vue#L124

The field should be not required and maybe make this explicit (e.g. add label "Only required for organizations registered in the Netherlands, else leave empty")