Add a computed-metadata.include for WG21

[brycelelbach]

• Allows the URL metadata field to be omitted - if it is, the link is computed from the paper and revision number using wg21.link.

[jfbastien]

Oooh nice!

[tabatkins]

Note that this isn't "if omitted", it's "always" - computed-metadata has a higher precedence than in-document metadata.

(Another recent bug requested a way to do higher-priority in-document metadata for when you need computed-metadata for most things but occasionally want to override it, which I'll be adding soonish.)

So let me know if that's actually what you want and I'll merge.

[tabatkins]

ping @brycelelbach !

[brycelelbach]

Hrm. @jfbastien what do you think?

I'm inclined to say it's fine and we should go ahead and merge it.

[jfbastien]

Yeah that works, it’s always the right value, even for a draft. Can you make sure WG14 doesn’t get the same thing though?

[tabatkins]

Are y'all still interested in this, or should I close it?

[jfbastien]

I think Bryce should do it.