Use default prechecked state on page load

DO NOT MERGE: 
Depends on https://github.com/alphagov/finder-api/pull/92
Without the above change this will not break finder frontend but it will remove the default preselection for drug-device-alerts

Rather than always prechecking select boxes, use the prechecked state from the metadata about an email signup currently pulled from finder-api.

Ticket: https://trello.com/c/DzJdcftL/431-checkbox-email-signup-for-other-finders-3