HTML export: Export #+TITLE as h1 and offset other headlines accordingly

emacs exports #+TITLE as <h1> and all other headlines accordingly as
`lvl + 1` - we'll go with that.

org/testdata/footnotes.html

@@ -6,9 +6,9 @@
 </li>
 </ul>
 </nav>
-<h1 id="headline-1">
+<h2 id="headline-1">
 Using some footnotes
-</h1>
+</h2>
 <ul>
 <li>
 <p>
@@ -42,9 +42,9 @@ footnotes that reference a non-existant definition are rendered but log a warnin
 </p>
 </li>
 </ul>
-<h1 id="headline-2">
+<h2 id="headline-2">
 Footnotes
-</h1>
+</h2>
 <p>
 Please note that the footnotes section is not automatically excluded from the export like in emacs. <sup class="footnote-reference"><a id="footnote-reference-8" href="#footnote-8">8</a></sup>
 </p>