Removed trailing `/site` from nightlies update-sites

Our nightlies update-sites don't follow the same convention of the ecosystem
update-sites. In particular, nightlies update-sites don't have a trailing
`site`.

With the intent of avoiding duplication, I've added a new variable
``fullDownloadUrl`` in `download-box.txt`. ``fullDownloadUrl`` is the actual
update-site url getting displayed in the produced html page.