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/install/install.go

@@ -559,7 +559,7 @@ func SubmitInstall(ctx *context.Context) {
 		}
 
 		days := 86400 * setting.LogInRememberDays
-		ctx.SetCookie(setting.CookieUserName, u.Name, days)
+		ctx.SetSiteCookie(setting.CookieUserName, u.Name, days)
 
 		ctx.SetSuperSecureCookie(base.EncodeMD5(u.Rands+u.Passwd),
 			setting.CookieRememberName, u.Name, days)