Populate user email address in newsletter signup form

If a user's email address is known (i.e. on apps, user is logged in), they should not have to enter their email address in the newsletter signup form.

This currently works on the web:

But not in apps:

Following this implementation, make sure to test it works in the app on device or in a simulator.

There is an Asana ticket for native changes here:

https://app.asana.com/1/1210045093164357/project/1215309367148854/task/1215837316731759

Native PRs:

- guardian/ios-live#10072
- guardian/android-news-app#13238

Review the linked Asana ticket and the native PRs guardian/ios-live#10072 and guardian/android-news-app#13238 before starting, then compare their behavior with the web implementation shown in this issue. Done means a logged-in user’s known email is populated in the newsletter signup form on the app, verified on a device or simulator.

Written by the indexing model from the issue text.