Use globally shared HTMLRender (#24436)

The old `HTMLRender` is not ideal.

1. It shouldn't be initialized multiple times, it consumes a lot of
memory and is slow.
2. It shouldn't depend on short-lived requests, the `WatchLocalChanges`
needs a long-running context.
3. It doesn't make sense to use FuncsMap slice.


HTMLRender was designed to only work for GItea's specialized 400+
templates, so it's good to make it a global shared instance.

routers/install/routes.go

@@ -66,7 +66,7 @@ func installRecovery(ctx goctx.Context) func(next http.Handler) http.Handler {
 					if !setting.IsProd {
 						store["ErrorMsg"] = combinedErr
 					}
-					_, rnd := templates.HTMLRenderer(ctx)
+					rnd := templates.HTMLRenderer()
 					err = rnd.HTML(w, http.StatusInternalServerError, "status/500", templates.BaseVars().Merge(store))
 					if err != nil {
 						log.Error("%v", err)