Reset scores

Add a new function `reset_scores()` to the Submissions API to allow the LMS instructor dash to reset a specific student's scores on a problem.

The basic approach is:
* The LMS retrieves scores from submission API calls that query the `ScoreSummary` model, which is mutable, redundant, and incrementally updated.
* By deleting the `ScoreSummary` model, we can make it appear as if a student's scores were reset without modifying the `Submission` or `Score` models.
* In order to track which scores are included in the summary, I added a `first_score_timestamp` field to the score summary model.  This defaults to the minimum datetime, indicating that current score summaries should include all scores currently in the database (which is true, since until now it was impossible to delete score summaries). 

@stephensanchez @ormsbee