Fix cascading delete

### Summary

This fixes the problem where deleting an organization did not cascade delete some of the other models. I merged #124 into this to not break the tests when they merge so please review that one first :smile: 

### Files changed
- app.py: Modified models to setup cascading
- factories.py: Fixed `IssueFactory.project_id` field
- tests.py: Added test_cascading_delete to test deletions