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.