Eliminate whitespace comments from generated pages

Every generated page contains a fragment like this:

<!-- ========= END OF TOP NAVBAR ========= -->
<span class="skip-nav" id="skip.navbar.top">
<!-- -->
</span></nav>
</header>

The <span> element contains both a newline and a comment and needs neither. They should both be removed.

The comment was originally used as a "workaround" to prevent the <span> element being dropped for being empty by the internal HTMLTree.add code. We now have better ways to manage that.