Form component should have a disabled prop, which disables submission even with the Enter key

[davidtheclark]

We recently had a bug where we disabled the FormSubmit component (using its disabled prop), but the Form could still be submitted with the Enter key. To avoid this situation, there should be a disabled prop on the Form component, which we could have set to true.

(In the meantime, the workaround is to do nothing in the handleFormData callback when the form should be disabled.)