Enhancement: Improve Stats for Viewing Subscriber Engagement

### Impacted plugin

Jetpack

### What

A user contacted us wanting to see improved views of subscriber engagement over time.

### How

User wants to have more options to compare stats over time. Their users are transitioning towards subscriptions, so they'd like to be able to compare clicks and subscription numbers over time:

> - compare them to subscription post numbers over a long history
> - compare total engagements (link clicks and email opens) to see if my total audience is declining, or if it's still increasing.

The issue names no files, tests, or entry points, so first locate Jetpack’s existing subscriber-engagement stats implementation and comparison views. Clarify the desired long-history subscription comparison and the total-engagement calculation; done should provide comparisons for subscription posts, link clicks, and email opens over time.

Written by the indexing model from the issue text.