Receipts Emails: Add User Friendly Description

### What

Add a more detailed description of the purchase at the top, where it's easier to spot.

### Why

In support, we receive a lot of email replies to the receipt emails, especially renewals, with questions that can be found on the user's account, without needing to contact support. Here's an example: 4216917-zd-woothemes

### How

This is what we currently send to users:


Details about the purchase are only available in a formal way on the receipt, maybe we could add a more user-friendly description of the purchases at the top. Here's an idea:


No source file, test, or entry point is named in the issue. Start by locating the receipt-email implementation in wp-calypso and compare the current and proposed email examples; done means purchase details are presented in a clearer, more visible description without losing the formal receipt information.

Written by the indexing model from the issue text.