New analytic: MRR

We've at long last added support for viewing MRR alongside your other analytics within Buttondown.

MRR is a tricky metric to calculate (our documentation outlines some of the reasons why), but it's also an important one. We had to build a new system to calculate it that ingests your entire Stripe history — which also means we can calculate it for newsletters that have been around for years, on other platforms and pricing models.