Refactor cookie (#24107)

Close #24062

At the beginning, I just wanted to fix the warning mentioned by #24062

But, the cookie code really doesn't look good to me, so clean up them.

Complete the TODO on `SetCookie`: 

> TODO: Copied from gitea.com/macaron/macaron and should be improved
after macaron removed.

routers/web/auth/password.go

@@ -336,7 +336,7 @@ func MustChangePasswordPost(ctx *context.Context) {
 
 	log.Trace("User updated password: %s", u.Name)
 
-	if redirectTo := ctx.GetCookie("redirect_to"); len(redirectTo) > 0 && !utils.IsExternalURL(redirectTo) {
+	if redirectTo := ctx.GetSiteCookie("redirect_to"); len(redirectTo) > 0 && !utils.IsExternalURL(redirectTo) {
 		middleware.DeleteRedirectToCookie(ctx.Resp)
 		ctx.RedirectToFirst(redirectTo)
 		return