Fixing slack integration

28e1148 changed SlackService notifier to rely only on the service_url, and no longer needing an api_token

This apparently broke the form for any newly created app, because a notifier was only considered `configured?` if the api_token was set, which is now impossible to do for slack

I wasn't really sure how to go about this, because I'm not very familiar with the codebase yet, but I just thought I'd push the fix I've made on my fork so this can be properly discussed

app/models/notification_services/slack_service.rb

@@ -24,4 +24,8 @@ def post_payload(problem)
   def create_notification(problem)
     HTTParty.post(service_url, :body => post_payload(problem), :headers => { 'Content-Type' => 'application/json' })
   end
+
+  def configured?
+    service_url.present?
+  end
 end