- Add monthly-submissions endpoint to the UserProfiles viewset which returns
  the total number of submissions in a given month to the user's forms
- Add monthly submissions serializer
- Add tests

- Change query to be more efficient
- Optimize code in serializer

- Add validation for the query parameters in the monthly submissions endpoint
- Return first item in the list, which is the dictionary with the total number of submissions
- Change the tests to check for a dictionary instead of a list of a dictionary