Add nits_receieved page

* Add acceptance tests for nits
* Link to nits receieved & given from nitstats page, not account page
* Standardize API endpoints:
  /nits/:username/stats
  /nits/:username/given
  /nits/:username/received
* Move ActiveRecord scopes from Routes to Comment model.
* For nits given, say: "Given to #{nitpick.submission.user}"
* For nits received, say: "Receieved from #{nitpick.user}"