Issue fixes

### Summary

I updated `Issues` to get deleted when it's parent `Project` gets deleted. I also added some test coverage for issues.

#### app.py
- Set single_parent and cascade on project property in Issue model
- Set ondelete Cascade on project.id property in Issue Model

#### run_update_test.py
- Modify Issue Mock to return a JSON containing sample Issues
- Modify test_import to also verify that Issues are added.
- test_main_with_good_new_data to also verify that bad Issues are deleted.

@migurski and @ondrae: This should fix the Engine Light complains.