Pass feed URL straight through to govukdelivery.

This greatly simplifies the process of creating
email signups.

Rather than passing params to a form which sends
options a controller which then regenerates an
atom URL by parts, just pass the URL straight
through and ask the user only for a confirmation
of their subscription (with local government
opt-in).
Properly build and case document feed URL descriptions.
Properly describe non-document alerts

e.g. policies, people, roles
Refactor govuk delivery notification.

- Switched to Sidekiq
- GovUkDeliveryEndpoint:
  - extracted SubscriptionUrlGenerator
  - extracted EmailFormatter
  - renamed the remaining class to 'Worker' reflecting the fact that it is now
    only a simple wrapper for the async job.
- Removed the explicit autoloading configuration and instead rely
  upon Rails conventions.
Strip out the group from the options arrays because Rails.