Report coverage stability in the dashboard

[Zac-HD]

We should also report coverage stability fraction, including a rating stable (100%), unstable (85%--100%), or serious problem (<85%"); and explain the difference between stability (=coverage) and flakiness (=outcome). Stability is mostly an efficiency thing; flakiness means your test is broken.

This mostly requires measuring both of these on the backend and then plumbing the data around, it's not hugely involved.