HTML export uses anchor ids which change on every export

[sbaugh@catern.com]

HTML export wraps headlines in anchor tags with IDs, so that they can be
linked by suffixing #[anchor-tag-ID] to the URL.

HTML export used to use anchor IDs like "sec-2" for the second headline,
but at some point it switched to generated IDs like "org7ffb324", which
change on every re-export.

This means anchor-links on external sites (that is, links which link to
a specific section of an org file) break every time an org file is
re-exported to HTML. The old style of anchor IDs would break URLs when
sections moved around, but at least it wouldn't break on every
re-export!

This makes org much less useful for typical web publishing use cases.

This can be worked around by setting CUSTOM_ID for every headline, which
will override the anchor id used, but I think it was much better when it
just worked by default...

It looks like this was changed in commit
459033265295723cbfb0fccb3577acbfdc9d0285
"Export back-ends: Use `org-export-get-reference'"

Perhaps this functionality (of generating anchor IDs based on the
section number) could be added back in?