Create App for Testing

Ass discussed in #8, tests will now use an app instead of mock.

- [ ] Move tests into `test_tracking.py` and `test_redirects.py` and `test_integration.py`
- [ ] Create `models.py` for testing
- [ ] Create `urls.py` for testing
- [ ] Add app to `INSTALLED_APPS`
- [ ] Refactor `test_tracking.py`