Extensions page from trisquel.info

[Rubén Rodríguez]

I've modified our extensions page to make it fit better in the browser,
so now it can be accessed with this url:
http://trisquel.info/en/browser-plain

The page is generated the same way as the normal site (without
"-plain"), so adding new extensions, screenshots or translations can be
done with a Trisquel user account as usually. I still have to improve
the css a bit, but feel free to use it for IceCat if you like.

Also, suggestions are welcome :)