PB-1112: Use pytest

[msom]

This PR improves the test suite, which previously contained a mix of Django and pytest tests. It

• moves all tests to app/tests to align with the structure of the other services
• removes all __init__.py files within test folders to prevent test code from being auto-discovered outside of pytest
• transfers the functionality of utils/testing.py to fixture(s) in conftest.py
• adapts the tests to use pytest

[schtibe]

Very nice, thank you!

I have made some comments along the reviewing concerning the naming of things. It wasn't always a 100% apparent what is what, maybe this gives you a hint for improvements.

[msom]

Your right, naming was confusing! Cleanup done:

• Changed all the data fixtures to individual fixtures returning single database objects
• Renamed user_factory to django_user_factory
• Moved the functionality of cognito_user to a global fixture cognito_user_response_factory
• Documented the global (factory) fixtures

[schtibe]

Nice, you made it even better! Thank you!