[CHANGE] Runestone database identifiers.

[Rob Beezer]

@label, as an alternative to @xml:id, had been mooted for some time, but it was
Runestone interactives, and their home in a database, that really forced the
issue. So hopefully everyone knows that a Runestone
component/interactive/exercise needs a (stable) @label identifier.

We had, for a while, a warning, if you did not have this. It only lasted five
days. It was buggy (and we didn't know why). Well, the subtle bug is now
fixed, and so the warning is back, and you might soon start seeing it (we hope
not!).

* You have @label - good for you, no warning.

* You have an (old) @xml:id, but no @label - tsk, tsk. But set a new @label to
the identical value as the @xml:id, and the warning will stop and the database
will be just fine. Carry on.

* No @xml:id, no @label - well, that is bad and you probably should have seen
some bad behavior already as a result.

Long sordid story at https://github.com/PreTeXtBook/pretext/pull/3082.

Rob