whatpr / PR Preview snapshots

[annevk]

Once #151 lands whatpr will still contain snapshots without the necessary warnings due to https://github.com/tobie/pr-preview/blob/2bc932997ebe66aebccb0f089f11d0f9d1840281/lib/wattsi-client.js#L75-L113 and due to whatwg/wattsi#64 not being supported for multipage.

Another problem with whatpr directly invoking wattsi is that various scripts from this repository are not applied so there will be a couple of subtle differences.

We should offer a build server that does include the html-build scripts and is able to emit multipage snapshots that include warnings.

[annevk]

@domenic my thinking here is that I will change the <default-or-review> argument to <default-or-multi-snap-or-review> which would only build a multipage snapshot.

And then we adjust build.sh so it also supports just building this, perhaps based on some environment variable.

And then you'd add support for that on the server, e.g., at /html-multi-snap.