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.

modules/context/context.go

@@ -677,7 +677,7 @@ func getCsrfOpts() CsrfOptions {
 
 // Contexter initializes a classic context for a request.
 func Contexter(ctx context.Context) func(next http.Handler) http.Handler {
-	_, rnd := templates.HTMLRenderer(ctx)
+	rnd := templates.HTMLRenderer()
 	csrfOpts := getCsrfOpts()
 	if !setting.IsProd {
 		CsrfTokenRegenerationInterval = 5 * time.Second // in dev, re-generate the tokens more aggressively for debug purpose