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

[simison]

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:

[tanjoymor]

@simison Looks like Zap has this under control. I'm marking it as Triaged and Low Priority.

Feel free to set back to Needs Triaged if there's any action you need PAs to take.