Require at least one unit to be enabled (#24189)

Don't remember why the previous decision that `Code` and `Release` are
non-disable units globally. Since now every unit include `Code` could be
disabled, maybe we should have a new rule that the repo should have at
least one unit. So any unit could be disabled.

Fixes #20960
Fixes #7525

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: yp05327 <576951401@qq.com>

models/repo.go

@@ -40,7 +40,9 @@ var ItemsPerPage = 40
 
 // Init initialize model
 func Init(ctx context.Context) error {
-	unit.LoadUnitConfig()
+	if err := unit.LoadUnitConfig(); err != nil {
+		return err
+	}
 	return system_model.Init(ctx)
 }