Enhancement: Add List ID header to email subscriptions.

### Impacted plugin

Jetpack

### What

Adding the List-ID header to email subscriptions to enable filtering.

### How

I like to filter certain email subscriptions via wordpress.com -- mainly emails from make.wordpress.org as I subscribe to a number of them and it can be a lot.

The recent removal of "[New Post]" from Jetpack emails broke a number of my filters.

Including a List ID header would allow for filters that are 100% accurate. Similar to the headers used in Github notifications.

I'd like a list ID header to be added to these emails:

```
List-ID: Blog name
```

Where blog ID is the unique identifier of either the Jetpack linked self hosted account or the WordPress.com blog ID (presuming these use the same numbering space).

Start by locating the Jetpack email subscription generation entry point and inspect how existing notification headers are assembled. Trace how the linked self-hosted account or WordPress.com blog ID is available, then verify that subscription emails contain the requested List-ID value and that filtering remains accurate.

Written by the indexing model from the issue text.