I have already signed the copyright assignment. (Though I used my
other email rudiwillalwaysloveyou@gmail.com when signing it.)

I have manually tested the changes with a custom value for
`org-html--id-attr-prefix', and everything works.

I did not change the behavior of `org-html-prefer-user-labels'; the ID
property will not be used if `org-html-prefer-user-labels' is nil. IMO
this behavior should be changed, and it should be always used, just
like CUSTOM_ID.

Changelog:

* org/ox.el (org-html--id-attr-prefix): Refactor hardcoded "ID-" as a
new private variable.

* org/ox-html.el (org-html--reference): Add support for the ID
property (previously only supported CUSTOM_ID)

These changes make crossfile ID links work in the exported HTML when
`org-html-prefer-user-labels' is not-nil.