fix #1710: ui: storage: add download from url button

fix #1710: ui: storage: add download from url button

uses the common function PVE::Tools::download_file_from_url to
download a iso image or container template.

note: Only users with permissions `Sys.Audit` and `Sys.Modify` on
`/` are permitted to use the api endpoints due to security reasons.
(it is possible to download files from internal networks which would
be not visible/accessible from outside)

Signed-off-by: Lorenz Stechauner <l.stechauner@proxmox.com>
Reviewed-By: Dominik Csapak <d.csapak@proxmox.com>