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.
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.