Add WHATWG Commit Snapshot computed metadata

[annevk]

@tabatkins api.csswg.org picks this up immediately, right? We need to change our invocation of that and I wonder how we need to balance things.

cc @domenic

[domenic]

This could go further, updating computed-metadata.include to replace [SNAPSHOT-LINK] with a line that uses [COMMIT-SHA]. Then we could remove all the lines in the deploy script that deal with SNAPSHOT-LINK.

Finally we could also move the warning stuff here by adding a header-LS-COMMIT, similar to what we did for header-RD.

[annevk]

Duplicating the whole header to remove the need for md-warning="Commit $SHA $COMMIT_URL_BASE$SHA replaced by $LS_URL" seems like the wrong direction to me.

[domenic]

Hmm. I like the way it keeps things centralized, but I see what you mean. There's probably also a way of avoiding the duplication, but I can try to make that work on top of this.

[domenic]

OK, I:

• Added back the links that I prefer to keep, since in IRC @annevk said he'd be OK with that
• Inlined the logo instead of keeping it in a separate file; this removes a wrapper <p>. This has nothing to do with the rest of the PR and is just something that has been bugging me slightly. I'm happy to remove it from this PR.
• Managed to get automatic warnings generated into the right place using the COMMIT-SHA. Improve Commit Snapshot setup whatwg/whatwg.org#310 is updated to take this into account. It stops using the "commit" warning type in favor of the "obsolete" warning type, because the commit warning type wants specific inputs, whereas with the "obsolete" warning type we can just use the COMMIT-SHA input alone. This feels kind of like a hack but also kind of elegant.

We should probably not merge this until all three of us are online and can sync the whatwg.org PR and discuss whether it's the best strategy. But, I'm pretty happy with it.

[annevk]

I cannot approve this, but I'm happy with it.

[annevk]

@domenic I checked with Tab and I can land this once we know what we want .pr-preview.json to look like. I also rebased the tests so CI will continue to pass.

[domenic]

OK, this is ready. It adds a new status, LS-PR, specifically for PR previews. This allows us to get a dedicated warning and h1 that talk about the PR, for example.

[annevk]

Update WHATWG metadata

To better account for Commit Snapshots and PR Preview.

See also https://github.com/whatwg/whatwg.org/pull/310 and https://github.com/whatwg/spec-factory/pull/5.