Add copy for metadata choices to drug-device-alerts

This adjusts the format of the metadata for the signup choices so each choice
can contain more information
Remove duplicate extra copy.

This extra copy ("email alert subscription") is added by `finder-frontend` as
it's common to all email subscriptions.

Don't add it here.
Merge pull request #83 from alphagov/copy-metadata-for-signup

Add copy for metadata choices to drug-device-alerts