Use format instead of slug for finding govdelivery url

In 5a107b7373cdc4e8fdf923b62b21d71027098016 we started using format
instead of slugs for the tags on email-alert-api.

This updates the getting of the subscription url to use format instead
of slug.
Merge pull request #121 from alphagov/use-format-for-email-subscription-finding

Use format instead of slug for finding govdelivery url