Fix N+1 queries on Nitpick page.

Hey! So, I ran rack-mini-profiler and bullet on the app to find performance issues.

Found the nitpick page had a couple N+1's.

So, to fix them I:

+ Added eager loading for ```user``` in ```submissions```.
+ Removed using ```user.nitpicker_on?``` in a loop. Because it was making 1 query per call.


**Fun stats:** 
http://localhost:4567/nitpick/javascript/word-count

*Before:* [(profiler screenshot)](https://www.evernote.com/shard/s10/sh/2ae1cc08-3bee-4d95-9c71-bdf5936bb03e/d395e4f16b2b22b5e9cbbe18b0072af9/res/6fd33807-8e43-4341-a2db-ec38c86ec0a2.jpg?resizeSmall&width=832)
~ 112 queries. 
~ 700ms

*After*: [(profiler screenshot)](https://www.evernote.com/shard/s10/sh/a8754602-0bac-4425-9248-7b91ac0f0b98/6e0994d87b36c695d6cdf0c420cdf4d4/res/794e8d93-a1fb-414a-9a38-37aaeb821452.jpg?resizeSmall&width=832)
~ 12 queries. 
~ 100ms