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/server/config.rs

@@ -1,7 +1,9 @@
 use std::collections::HashMap;
 use std::path::{PathBuf};
+use anyhow::Error;
 
 use hyper::Method;
+use handlebars::Handlebars;
 
 use proxmox::api::{ApiMethod, Router, RpcEnvironmentType};
 
@@ -10,17 +12,22 @@ pub struct ApiConfig {
     router: &'static Router,
     aliases: HashMap<String, PathBuf>,
     env_type: RpcEnvironmentType,
+    pub templates: Handlebars<'static>,
 }
 
 impl ApiConfig {
 
-    pub fn new<B: Into<PathBuf>>(basedir: B, router: &'static Router, env_type: RpcEnvironmentType) -> Self {
-        Self {
-            basedir: basedir.into(),
+    pub fn new<B: Into<PathBuf>>(basedir: B, router: &'static Router, env_type: RpcEnvironmentType) -> Result<Self, Error> {
+        let mut templates = Handlebars::new();
+        let basedir = basedir.into();
+        templates.register_template_file("index", basedir.join("index.hbs"))?;
+        Ok(Self {
+            basedir,
             router,
             aliases: HashMap::new(),
             env_type,
-        }
+            templates
+        })
     }
 
     pub fn find_method(