Make the default admin group code a constant

We should consider locking this record attribute in future?

modules/backend/database/seeds/SeedSetupAdmin.php

@@ -27,7 +27,7 @@ class SeedSetupAdmin extends Seeder
     {
         $group = UserGroup::create([
             'name' => 'Owners',
-            'code' => 'owners',
+            'code' => UserGroup::DEFAULT_CODE,
             'description' => 'Default group for website owners.',
             'is_new_user_default' => false
         ]);

modules/backend/models/UserGroup.php

@@ -10,6 +10,11 @@ use October\Rain\Auth\Models\Group as GroupBase;
  */
 class UserGroup extends GroupBase
 {
+    /**
+     * @var string The default group code.
+     */
+    const DEFAULT_CODE = 'owners';
+
     /**
      * @var string The database table used by the model.
      */