style.css,html.py: use <pre> instead of <br/>; re.sub instead of re.finditer

After looking into it some more, I think I've figured out how to handle <pre>
in the CSS. So, use that, when description has more than one line.

Also, go to re.sub for turning URLs into links. The version of Python I was
testing my code on wasn't properly handling backreferences in the replacement
string when in the form "\#", causing me to use finditer instead of sub. But
I've discovered that it does handle backreferences in the form "\g<#>". So
switch to much simpler re.sub code.
html.py: replace Windows backslashes in icon paths

This is one source of missing-image results.

There remain other reasons for missing icons. The script doesn't find images
in add-ons. And when resources are moved or renamed, they are no longer found
by the script, even if they had been found before.

Also, capitalize a sentence.