Add tests on json export

.gitlab-ci.yml

AKDashboard/tests.py

AKModel/availability/models.py

AKModel/fixtures/model.json

AKModel/tests/__init__.py

AKModel/tests/test_json_export.py

AKModel/tests/test_views.py

AKModel/views/ak.py

AKPlan/tests.py

AKScheduling/api.py

AKScheduling/models.py

AKScheduling/tests.py

AKSubmission/tests.py