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 "Visible To" hidden does nothing.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14812