Add query parameter to order API results explicitly

[TimJentzsch]

Relevant issue: Closes #172

Description:

This uses the OrderingFilter general filter back-end to provide an ordering query parameter. This can be used to explicitly control the ordering of the API results.

For example, /api/submission/?ordering=complete_time and api/submission/?ordering=-complete_time will return the submissions filtered by the time they were completed at, ascending and descending respectively.

I added orderings to the submission, transcription and volunteer endpoints.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [x] Tests (if applicable)
• [x] Success Criteria Met
• [x] Inline Documentation
• [ ] Wiki Documentation (if applicable)