Use "manage form display" for subscriber base fields

Some of this change was already committed as part of 🐛 Model for confirmations is flawed Fixed .

The last submitted patch, 2: simplenews.subscriber-form-dislpay.3375320-2.patch, failed testing. View results →

Unfortunately setting the langcode entity key has side effects (all existing subscriptions didn't show up because they had the wrong langcode), so undo that part.

Automatically closed - issue fixed for 2 weeks with no activity.