fix(issue): prevent redirect to external tracker when built-in tracker is enabled (#39997) (#40031)

Backport #39997 by petervavrinecz

Signed-off-by: Vavrinecz Péter <98645804+petervavrinecz@users.noreply.github.com>
Co-authored-by: petervavrinecz <98645804+petervavrinecz@users.noreply.github.com>

routers/web/repo/issue.go

@@ -100,7 +100,7 @@ func MustEnableIssues(ctx *context.Context) {
 	unitExtTracker, err := ctx.Repo.Repository.GetUnit(ctx, unit.TypeExternalTracker)
 	if err == nil {
 		extURL := unitExtTracker.ExternalTrackerConfig().ExternalTrackerURL
-		if extURL != "" {
+		if extURL != "" && !ctx.Repo.Permission.CanRead(unit.TypeIssues) {
 			ctx.Redirect(extURL)
 			return
 		}