R1:ed43b31cb122

Prevent "Spaces" field from being set to inconsistent values

Summary:
At least for now, the "Space" field is just a subfield of the "Visible To" field, so:

- it doesn't get any separate settings; and
- it always uses the "Visible To" settings.

Test Plan:
- Created a form with a hidden view policy field.
- Created stuff with no "you must pick a space" errors.
- Created stuff with a normal form.
- Prefilled "Space" on a noraml form.
- Verified that trying to prefill "Space" on a form with…