Refactor external URL detection (#29973)

Follow #29960, `IsExternalURL` is not needed anymore. 
Add some tests for `RedirectToCurrentSite`

routers/utils/utils.go

@@ -5,26 +5,10 @@ package utils
 
 import (
 	"html"
-	"net/url"
 	"strings"
-
-	"code.gitea.io/gitea/modules/setting"
 )
 
 // SanitizeFlashErrorString will sanitize a flash error string
 func SanitizeFlashErrorString(x string) string {
 	return strings.ReplaceAll(html.EscapeString(x), "\n", "<br>")
 }
-
-// IsExternalURL checks if rawURL points to an external URL like http://example.com
-func IsExternalURL(rawURL string) bool {
-	parsed, err := url.Parse(rawURL)
-	if err != nil {
-		return true
-	}
-	appURL, _ := url.Parse(setting.AppURL)
-	if len(parsed.Host) != 0 && strings.Replace(parsed.Host, "www.", "", 1) != strings.Replace(appURL.Host, "www.", "", 1) {
-		return true
-	}
-	return false
-}