add Slack API webhook support

conf/locale/locale_en-US.ini

@@ -234,6 +234,11 @@ settings.update_webhook = Update Webhook
 settings.update_hook_success = Webhook has been updated.
 settings.delete_webhook = Delete Webhook
 settings.recent_deliveries = Recent Deliveries
+settings.hook_type = Hook Type
+settings.add_slack_hook_desc = Add <a href="http://slack.com">Slack</a> integration to your repository.
+settings.slack_token = Token
+settings.slack_domain = Domain
+settings.slack_channel = Channel
 
 [org]
 org_name_holder = Organization Name