Add download link to downloaded file

This adds a simple download link that points to the downloaded file.

Note: This makes all files in the download directory (and its
sub-directory) available to any user.

Closes gh-26

ui/src/app/downloads.service.ts

@@ -15,6 +15,7 @@ interface Download {
   url: string,
   status: string;
   msg: string;
+  filename: string;
   quality: string;
   percent: number;
   speed: number;