Merge branch 'sqlalchemy' into 'master'

.gitlab-ci.yml

migrations/alembic.ini

migrations/env.py

migrations/script.py.mako

migrations/versions/4204f4a83863_pony_to_sqlalchemy.py

package.json

requirements.txt

tests/test_database.py

tests/test_scans.py

zesje/__init__.py

zesje/api/emails.py

zesje/api/exams.py

zesje/api/export.py

zesje/api/feedback.py

zesje/api/graders.py

zesje/api/images.py

zesje/api/problems.py

zesje/api/scans.py

zesje/api/signature.py

zesje/api/solutions.py