Add custom templates to /api/get_templates_listing

This has to do with https://github.com/designcreateplay/NodeBB/issues/849, https://github.com/designcreateplay/NodeBB/pull/846 and https://github.com/designcreateplay/NodeBB/issues/765

This adds custom_routes.templates to to /api/get_templates_listing. This allows plugins to ajaxify their pages (@psychobunny)