Subscriptions: ask for subscription confirmation before subscribing unverified-email account

When subscribing to a site at `*.wordpress.com` and simultaneously logged in at WP.com with account which hasn't verified their email, we should send "confirm your subscription" email which also confirms their email address.

- Create an account and don't verify its email https://wordpress.com/start/user-social
- While logged in at WP.com, subscribe at https://cicerosletters.wordpress.com/
- You successfully subscribe

Instead we should show "Go to your email to confirm your subscription".

If you attempt to subscribe from the Reader, we show a nudge to verify your email:

Reproduce the flow from wordpress.com/start/user-social, then subscribe at cicerosletters.wordpress.com while logged in with an unverified account; compare it with the Reader behavior shown in the issue. Trace the subscription and email-verification entry points, and consider the work done when the site flow shows “Go to your email to confirm your subscription” and sends the confirmation email.

Written by the indexing model from the issue text.