:ocean: Make recent submissions more likely to be picked

* Make the likelihood of a submission being randomly chosen for
  nitpicking deteriorate over time.
* Give a 30% chance that an exercise other than the last one you
  completed will be chosen as one for you to nitpick.
* No longer take number of nits into account when selecting an exercise.

This will close #965
Merge pull request #997 from rsslldnphy/feature/decay

Make recent submissions more likely to be picked