Add links between nits pages

As @kytrinyx mentions in #2040:

> Perhaps we can add links from the nits received page to the nits given and nit stats pages, and same for the nits given page.

* There are only tests for the links on the nitstats page - not for the links on the given or received pages.  Should I add tests for those? Or is that overkill?

* Any input on the html markup / design / styling of the links is definitely welcome!

Nit stats:
![nit_stats](https://cloud.githubusercontent.com/assets/5103414/5236070/9b137530-77ee-11e4-9be6-86f94cefd1c4.png)

Nits received (given is similar):
![nits_received](https://cloud.githubusercontent.com/assets/5103414/5236072/a83c1b0e-77ee-11e4-82f3-b34145339125.png)