Add configuration option to hide official templates

Introduce a new environment variable and configuration option to allow users to hide official templates in the template gallery. This provides more flexibility for self-hosted instances of OpnForm.

api/app/Http/Controllers/Forms/TemplateController.php

@@ -14,7 +14,7 @@ class TemplateController extends Controller
 {
     private function getProdTemplates($slug = false)
     {
-        if (!config('app.self_hosted')) {
+        if (!config('app.self_hosted') || !config('opnform.show_official_templates')) {
             return [];
         }