Retry transaction if there are conflicts, Closes #28

api/config/api_example_config.py

api/src/api/announcement.py

api/src/api/authentication.py

api/src/api/course.py

api/src/api/database/__init__.py

api/src/api/database/abstract_py_connector.py

api/src/api/database/database.py

api/src/api/database/mysql_connector.py

api/src/api/database/postgres_connector.py

api/src/api/database/sqlite_connector.py

api/src/api/job.py

api/src/api/objects/current_changelog.py

api/src/api/objects/object.py

api/src/api/routes/courses.py

api/src/api/routes/object_modifications.py

api/src/api/routes/site.py

api/src/api/routes/sorter.py

api/src/api/routes/user.py

api/src/api/user.py