Added config option for using Sentry for user authentication

Adding the service provider was the only way I could get the config file to read. Not sure if that's the best way to do it, but it works well. 

I also added a separate getUserId() function to reduce the complexity of the afterSave() function. Thought having it called outside of the foreach loop would help performance too since it would not have to check each loop.

Also, I though having a config file would also open up other options in the future.