* contrib/lisp/ox-freemind.el: New file.
* lisp/ox-html.el (org-html--tags, org-html-format-headline)
(org-html--format-toc-headline, org-html-checkbox)
(org-html-table-cell, org-html-timestamp)
(org-html-verse-block, org-html-special-string-regexps):
Replace named HTML entities with their numeric counterparts.
This keeps Freemind backend happy.