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/package.go

@@ -131,7 +131,7 @@ func determineAccessMode(ctx *Context) (perm.AccessMode, error) {
 
 // PackageContexter initializes a package context for a request.
 func PackageContexter(ctx gocontext.Context) func(next http.Handler) http.Handler {
-	_, rnd := templates.HTMLRenderer(ctx)
+	rnd := templates.HTMLRenderer()
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(resp http.ResponseWriter, req *http.Request) {
 			ctx := Context{