Titlecase alert types for MSA

Ticket:
https://trello.com/c/9zAer1Z4/491-lowercase-medical-device-alert-and-drug-alert
Merge pull request #100 from alphagov/lower-case-msa-alert-types

Titlecase alert types for MSA
Move drug device alerts to new content format

By no longer supplying a key in the email_signup_choices hash, and
making it an array, we can simplify a lot of our code.

This commit also renames ‘name’ to ‘radio_button_name’, to distinguish
it from the ‘topic_name’, which has been added. This is due to
sometimes wanting to display something different in the topic title of
a subscription from what was on the radio button.
Add subscription_list_title_prefix to signup content

New option to add a prefix to a subscription’s subscription list title.
Ensure it is always present.
Add subscription options to AAIB reports

Using the new subscription_list_title_prefix option, this adds 5 ways
to filter your subscription to AAIB reports.
Remove rake task to precreate email lists

This is now handled by finder-fronted
Merge pull request #101 from alphagov/better-susbcription-options

Better subscription options
Re-add filter key

Tom was right, I was wrong. We need this key to match up with
Specialist Publisher’s tagging.
Merge pull request #102 from alphagov/readd-filter-key

Re-add filter key