luci-base: use cgi-io and rpcd-mod-file to handle file upload and browsing

Remove the old server side support for file browsing and file uploading
and switch to a client side widget instead which uses XMLHTTPRequests to
upload files via cgi-io and RPC calls for file listing and status queries.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>