React hydration error in Next.js development server

[jbirch]

This is entirely the same as igrigorik#1160 and igrigorik#1255, as it shares the same root cause that lives over in https://github.com/codebicycle/videospeed/blob/firefox-port/inject.js#L538 here.

Injecting a className on the body causes us to run afoul of facebook/react#24430, in which React notices that the HTML it thinks should exist and the HTML that actually does exist is sufficiently different that it refuses to do a hydration.

Unfortunately, we've drifted pretty far from igrigorik/videospeed, and their injection logic is significantly different. It looks like it currently sets a style on the html tag instead of --vsc-domain: ....