change index to templates using handlebars

using a handlebars instance in ApiConfig, to cache the templates
as long as possible, this is currently ok, as the index template
can only change when the whole package changes

if we split this in the future, we have to trigger a reload of
the daemon on gui package upgrade (so that the template gets reloaded)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/bin/proxmox-backup-proxy.rs

@@ -34,7 +34,7 @@ async fn run() -> Result<(), Error> {
     let _ = csrf_secret(); // load with lazy_static
 
     let mut config = ApiConfig::new(
-        buildcfg::JS_DIR, &proxmox_backup::api2::ROUTER, RpcEnvironmentType::PUBLIC);
+        buildcfg::JS_DIR, &proxmox_backup::api2::ROUTER, RpcEnvironmentType::PUBLIC)?;
 
     // add default dirs which includes jquery and bootstrap
     // my $base = '/usr/share/libpve-http-server-perl';