How might we: Hide newsletter sign-up form if user has already signed up

If a user has signed up to an email newsletter, they may still see a form asking them to sign up for it.

This might require [a new endpoint](https://github.com/guardian/frontend/blob/58967e3555caac4181414cd071396a2b6842a28d/applications/conf/routes#L34-L50)

Start by inspecting applications/conf/routes around the linked lines and locating the newsletter sign-up form in the rendering service. Trace how a user's existing newsletter subscription could be checked and determine the behavior for subscribed and unsubscribed users. Done means signed-up users no longer see the form while eligible users still can.

Written by the indexing model from the issue text.