Allow the user to sign up to the correct GovDelivery topic from finder-frontend signup

When choices exist for a finder
And the user selects a combination of options to sign up for
Or the user signs up to a finder without choices
Redirect the user to the correct govdelivery list for their choices.

**Testing notes**
If pulling down this branch to test it, note it will require deployment steps below *in addition* to adjusting the `email-alert-api` to connect to govdelivery stage environment using the correct credentials.

**Deployment notes**

Before this can be tested, the following apps need to be deployed in this order:

* Deploy `finder-api` to increase the timeout on the email list generator rake task.
* Ensure the `email_subscriptions:register_subscriptions` rake task completes successfully.
* Ensure the `publishing_api:publish` rake task completes successfully.
* Deploy the `email-alert-api` with this PR: https://github.com/alphagov/email-alert-api/pull/50