Add setting for paid subscriptions to only require payment for the email notification

### Impacted plugin

Jetpack

### What

Customer requested the ability to require their subscribers to pay for the subscription service of getting notified for new posts, without restricting the post visibility to paid subscribers only so that others could still view the post itself.

### How

Add a setting to the Newsletter settings page

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Start at Jetpack's Newsletter settings page and trace how paid subscriptions control post-notification access versus post visibility. Review the related subscription and notification behavior and any existing tests before defining the setting; done means subscribers can be charged for notifications while the posts remain visible to others.

Written by the indexing model from the issue text.