Added closed challenges endpoint and ported the recent challenges endpoint

A few notes:

1. I believe the `/challenges/closed` endpoint can be deprecated and removed in the future since it the `/challenges` endpoint handles the `open` option as a query string parameter.
2. I did not implement tests for the `/challenges/closed` endpoint because tests already exist for the `/challenges` endpoint. I can implement tests. I feel that it wouldn't test anything new except for the controller action.