Add `isnull` filters to endpoints

[TimJentzsch]

Relevant issue: Closes #201

Description:

This PR adds isnull filters to the endpoints, allowing clients to exclude or include null entries in the listings. This allows several new queries, such as getting all posts in the queue and getting all in-progress posts by a user.

For example, the submission endpoint now allows query parameters like ?completed_by__isnull=true which gets all submissions that have not been completed yet.

Checklist:

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