Add non-grace-period view on browser-specific APIs

@foolip says that in addition to the "1 year grace period" view of browser-specific APIs, he'd like to also see a picture of the debt we're creating the minute we are the first to ship a new API. I.e. at any given point in time, how many APIs exist for the one browser but no others.

I don't think this data will be particularly actionable (we don't consider shipping first to be problem in and of itself), but we should see a saw-tooth pattern that gives some indication of how we're creating temporary "stretch" in the band which later gets relaxed.