Fix "Save Torrent File" feature

This feature broke again after PR #8325. My fault for not catching it
during review. This PR fixes the code that is now on `master`.

Addresses these comments:

-
#8446 (comment)
-
#8146 (comment)
52

It is better to add the '?download=true' querystring param with
`url.parse` and `url.format` since blindly tacking on a
'?download=true' can lead to invalid URLs.

A couple examples:

magnet:?a=b&c=d?download=true should use &download=true instead
https://example.com/file.torrent#ix=1?download=true should not add a
query param after a hash

Also, this names the downloaded file correctly, which didn't seem to be
working either after PR #8325.

js/webtorrent/entry.js

@@ -178,10 +178,15 @@ function stop () {
 }
 
 function saveTorrentFile () {
-  let a = document.createElement('a')
+  const parsedUrl = url.parse(store.torrentId, true)
+  parsedUrl.query.download = true
+  const name = path.basename(parsedUrl.pathname) || 'untitled.torrent'
+  const href = url.format(parsedUrl)
+
+  const a = document.createElement('a')
   a.target = '_blank'
-  a.download = true
-  a.href = `${store.torrentId}?download=ok`
+  a.download = name
+  a.href = href
   a.click()
 }