Retrieve a newsletter's acquisition breakdown

Where a newsletter's new subscribers came from over a window: the split by the import that brought them in, and how the invite funnel performed.

by_source is about imports. It groups on the value an import stamped, so migrating a newsletter between providers later never relabels the history: a value that is already set is never overwritten. For whether somebody subscribed through Commune at all, read source on the subscriber, which is a different question with a different answer.

The counts here are arrivals Commune recorded inside the window and are not the newsletter's audience size. For a newsletter connected to an outside provider they also miss anyone who joined at the provider between two imports.