org-html: put the title back in the content div.
This is needed for the HTML file to interact correctly with
org-info.js. On the long term, the title should be part of
the preamble, but this requires work on org-info.js.
* org-html.el (org-export-html-preamble-format): New default
for the HTML preamble: don't include the title. Also improve
the docstring.
(org-export-html-postamble-format): Improve the docstring.
(org-export-as-html): Add the title within the "content" div.
This is necessary for interaction with the org-info.js script.