Fix custom field selector control in cases where the user doesn't edit anything

Summary: The JS and PHP representations of state can differ; just have the JS write the state out immediately on page load.

Test Plan: Saved diffusion.fields without making changes, reloaded, saw no effective change.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12998