Setting to automatically confirm subscriptions

#### Is your feature request related to a problem? Please describe.
When I'm replying to a blog post on a site with Jetpack Subscriptions enabled, I dislike that I need to confirm my subscription when I'm already logged in as a commenter.

#### Describe the solution you'd like

Create a setting at https://subscribe.wordpress.com/?option=settings that allows the confirmation step to be bypassed.

#### Describe alternatives you've considered

#### Additional context
I'm unsure if this can be abused. Not sure how often someone is spoofing another person's email address with subscription spam.

Start at the subscribe.wordpress.com/?option=settings entry point and trace the Jetpack Subscriptions confirmation flow for logged-in commenters. The issue names no repository file or test; done means the setting’s intended bypass behavior and the stated email-spoofing concern have been resolved.

Written by the indexing model from the issue text.