Added Endpoint to Get Submission Counts for Multiple Problems

@jbau @dsjen @dylanrhodes @rocha
from python docs: https://docs.python.org/2/library/itertools.html#itertools.groupby
```
The operation of groupby() is similar to the uniq filter in Unix. It generates a break or new group every time the value of the key function changes (which is why it is usually necessary to have sorted the data using the same key function)
```
I wonder if it's wise/necessary to order the `queryset` then?