Paid subscriptions: There is no confirmation modal when users cancel

### Impacted plugin

Jetpack

### What

When someone is a subscriver of a site (via a payment button, a paid newsletter, or paid content block), when they cancel the subscription tehre is no modal asking them to confirm and letting them know what the cancelation means.

### How

By adding a modal when someone chooses to cancel the subscription informing them they will lose access to the subscription and related content. We should also mention the site in question.

No file, test, or entry point is named. Start by locating Jetpack's subscription-cancellation flow and existing modal patterns; done means cancellation opens a confirmation modal explaining loss of access and related content and naming the site.

Written by the indexing model from the issue text.