Enhancement: Improve subscription payment confirmation email

### Impacted plugin

Jetpack

### What

Improve payment confirmation email when I pay for paid subscription

### How

If a reader goes to a site with a paywall block, and pays for access, they will get a payment confirmation email like the one below. Not sure if this email is generated by us or Stripe? But seems like it could be improved
- Issue 1: It says 'Automattic' at the top - not [WordPress.com](http://wordpress.com/) or the title of the specific site I paid to get access to.
- Issue 2: The email header says 'Thank you for subscribing to [Tier Name]! Maybe better to say 'subscribing to [Site Name]!'
- Overall design. We could improve the styling and use a design more in line with other emails we send out.

Current email looks like (not Cheap Tier refers to a plan name):

Start by tracing the Jetpack paywall block's payment confirmation email flow and determine whether the email is generated by Jetpack or Stripe. Compare the current sender branding, subscription heading, and styling with other emails; done means the responsible flow is identified and the requested branding and design improvements are specified.

Written by the indexing model from the issue text.