WEB: Fix download button background images again

We also need to take padding into account, which was omitted in the
previously updated images
WEB: Update the download button background colors to match the new images
WEB: Only show the recommended d/l button if a user agent match is found

Also, the creation of the button itself has been pushed to Javascript.
The recommended d/l section is now only shown if JS is enabled and an
appropriate user-agent has been matched