ox-html.el: Use CUSTOM_ID for outline-container-* div id
* lisp/ox-html.el (org-html-headline): When CUSTOM_ID is set, use it
to set the outline-container-* <div> id.
When exporting HTML the exporter generate ids for references.
Unfortunately those id are not stable in the sense that exporting
twice generate two different set of ids.
Using CUSTOM_ID one could already use have fixed anchors in the
generated HTML. So for example we could share URL with
...index.html#my-section-id and it will be the same URL even if we
export the HTML again.
Unfortunately, this CUSTOM_ID is not used for the outline ids. And
thus if we expose the html in a git repository for example. Each
export will generate a big diff that "pollute" the diff because it
will show changes for all org items while most of them could still be
With this PR, HTML export will use CUSTOM_ID for both the headers id
and the org outline ids. For Items without CUSTOM_ID the behavior will
stay identical as before.