html.py,style.css: use <pre> with no top-margin on all descriptions

After my last change dealing with this issue, I noticed that descriptions
with <pre> had an extra blank line at the top. Adding top-margin to the
CSS file made this go away, but it also made the <br/> superfluous. Thus,
it is simpler just to have every description use <pre> instead of <br/>.

This should finally resolve everything having to do with the add-on
descriptions.

If I'd noticed that the re module hadn't been imported, I probably wouldn't
have considered URL linking to be important enough to do so. Since I've
already written the code, however, I'll keep it.