This task requires you to install Newsletter extension.

So right now we have the subscribers of a newsletter being listed at Special:Newsletter/id/subscribers. The requirements of this task is to:

1. Make that field editable (this might involve implementing the submit function at https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/includes/specials/SpecialNewsletter.php#L495 )
2. Make sure things alter the settings in 'nl_subscribers' (like add in/ remove in the new user). You might want to check if the user exists for each entry etc. Most of this logic seems similar to the manage-publishers functioanlity in https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/includes/NewsletterEditPage.php#L279)