WEB: Add a "Recommended download" button to the downloads page

The idea has been taken and adapted from the VideoLAN web site
(http://www.videolan.org). A big 'download' button is shown,
which links to the most appropriate version for the user, depending
on the OS reported by the user's browser. Only OSes with a browser
have been added. Known browser user-agent strings have been taken
from http://user-agent-string.info/list-of-ua/os