add CSS file for PBS ExtJS6 basic ui

some fitting rules copied over from PVE's ext6-pve.css file.
simply place it in the css subfolder where the proxmox-backup-gui.js
file is hosted and add a "css/" alias for that directory, the
formatter gets use the right content type with that.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/bin/proxmox-backup-proxy.rs

@@ -47,6 +47,7 @@ async fn run() -> Result<(), Error> {
     config.add_alias("fontawesome", "/usr/share/fonts-font-awesome");
     config.add_alias("xtermjs", "/usr/share/pve-xtermjs");
     config.add_alias("widgettoolkit", "/usr/share/javascript/proxmox-widget-toolkit");
+    config.add_alias("css", "/usr/share/javascript/proxmox-backup/css");
 
     let rest_server = RestServer::new(config);