luci-base: ui.js FileUpload: option to enable Download button

Allow downloading from a file browser.
The Download button is located near to Delete.
It's shown only for files: folders or /dev/ devices can't be downloaded.
The downloading is made via fs.read_direct() which internally calls cgi-download.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>