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.
This line seems unnecessary, since you have both `"drug"` and `"device"` as keys in `"copy"` below. Might it be be better to change `"copy"` to `"choices"` and turn it into an array like:
```
"choices": [
  {
    "key": "devices",
    "name": "Medical device alerts",
    "body": "information published by the MHRA about defective medical devices."
  }
]
```