Some CSS to ensure that display math is
displayed centered and on a new line is now included
in the default HTML-based templates; this may be
overridden if the user wants a different behavior.
The web service passed in to `--webtex` may render formulas using inline
or display style by default. Prefixing formulas with the appropriate
command ensures they are rendered correctly.
This is a followup to the discussion in #5656.
