build: update Poetry dependencies: coverage, django-cors-headers, djangorestframework, flake8, jsonschema, pillow, pytest

Closes: 137

- Add `pytest-factoryboy` dependency
- Add pytest options
- Add factories for User, Host, Image, FundingCategory, Type, Show, RRule, Schedule and Timeslot
- Add pytest fixtures
- Add tests for Schedules, Shows, Timeslots ma Users

Replace ModelWithImageFields with foreign key to Image.

Closes: aura#164