Do not show newsletter sign-up form if user has already signed up (apps)

Use the Bridget method defined in guardian/bridget#262 to determine whether to show a newsletter sign-up embed.

Start by locating the apps newsletter sign-up embed in the dotcom-rendering repository and read the Bridget method referenced in guardian/bridget#262. Confirm how the embed identifies users who have already signed up. Done means the form is not shown to those users while remaining available to users who have not signed up.

Written by the indexing model from the issue text.