Add copy for metadata chocies to drug-device-alerts

Each checkbox/choice for an email signup needs associated copy, according to the prototype here: http://govuk-prototyping.herokuapp.com/mhra/drug_alerts.html

Adding this here makes the publishing_api:publish rake task push this to the content item for the signup page. meaning we can access it in finder-frontend.
Just seems like we should maybe take a similar approach to how the facets are enabled, since it would be nice to keep the processes similar for the future.