Add online commenting support

This patch adds support for online commenting. A new template
``com_form.html`` to customize the commenting form is added. Online
commenting is only available if ``enable_comments`` is True, and you need
to let the web server write to the ``comments_path`` directory.

Note that this patch exposes the race in the CommentDB.save() method
(described in the previous commit), but since this event is unlikely
enough, we don't care at the moment and can be fixed later if needed.