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/api/v1/misc/markup_test.go

@@ -30,7 +30,7 @@ const (
 )
 
 func createContext(req *http.Request) (*context.Context, *httptest.ResponseRecorder) {
-	_, rnd := templates.HTMLRenderer(req.Context())
+	rnd := templates.HTMLRenderer()
 	resp := httptest.NewRecorder()
 	c := &context.Context{
 		Req:    req,