Added get_top_submissions to submissions API

Added a new API to the submissions module which gets the top
submissions for the assessment
This function should validate `number_of_top_scores` and raise a `SubmissionRequestError` if:
* The number is < 1
* The number is greater than some maximum number of scores (maybe 100?).  The max number should be defined as a constant in this module, something like:
```
MAX_TOP_SUBMISSIONS = 100
```