Add: #363 特定サーバーの投稿にスタンプを付けることを禁止する管理者設定 (#364)

* Add: #363 特定サーバーの投稿にスタンプを付けることを禁止する管理者設定

* Fix test

app/controllers/admin/special_instances_controller.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+module Admin
+  class SpecialInstancesController < BaseController
+    def show
+      authorize :instance, :show?
+
+      @admin_settings = Form::AdminSettings.new
+    end
+
+    def create
+      authorize :instance, :destroy?
+
+      @admin_settings = Form::AdminSettings.new(settings_params)
+
+      if @admin_settings.save
+        flash[:notice] = I18n.t('generic.changes_saved_msg')
+        redirect_to after_update_redirect_path
+      else
+        render :show
+      end
+    end
+
+    private
+
+    def after_update_redirect_path
+      admin_special_instances_path
+    end
+
+    def settings_params
+      params.require(:form_admin_settings).permit(*Form::AdminSettings::KEYS)
+    end
+  end
+end