Support unsubscribe links

[mec]

Support for unsubscribe links is available on Notify now:

https://www.notifications.service.gov.uk/using-notify/unsubscribe-links

We should investigate how mail-notify can help support these.

API docs:

https://docs.notifications.service.gov.uk/ruby.html#one-click-unsubscribe-url-required

[asmega]

Please see #168