ox-html.el: Cleanup pre/postamble handling

ox-html.el: Cleanup pre/postamble handling

* lisp/ox-html.el (org-html--timestamp-format): New variable used
in the metadata and the pre/postamble.
(org-html-style-default): Make the preamble and postamble use the
same style.  Make all anchors font-size %100.  Remove margin from
the content section.
(org-html-container-element): Fix docstring.
(org-html-postamble-format, org-html-preamble-format): Update
docstrings.
(org-html-template): Use `org-html--build-pre/postamble'.
(org-html--build-pre/postamble): New function, combining the
pre/postamble generator.  Merge lists of formatters from the
preamble and postamble.  Fix bug, using :time-stamp-file instead
of :with-date for auto display of date: this brings usage in-line
with the latex and beamer exporter.
(org-html--build-postamble, org-html--build-postamble): Delete.