Add nits_receieved page

Add a "nits received" page, as per feature request in issue #2026


* Add acceptance tests for nits
* Link to nits received & given from nitstats page instead of from account page
* Standardize routes based on Katrina's suggestion:

  /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}"


This attempts to implement the "nits receieved" page with *as little change as possible* to the overall UI. Here are some things from the initial feature request that this *doesn't* implement because I'm not sure how:

>I would suggest to replace the Nit stats menu item in my profile menu with a Nits page. On this page, you should be able to see the latest nits received and written.

and

> and make this an angular app