Refactoring: rename covid19 to flask_covid19

src/flask_covid19/blueprints/app_all/all_model.py

src/flask_covid19/blueprints/data_ecdc/ecdc_model.py

src/flask_covid19/blueprints/data_owid/owid_model.py

src/flask_covid19/blueprints/data_rki_cases/rki_model.py

.../blueprints/data_rki_vaccination/rki_vaccination_model.py

src/flask_covid19/blueprints/data_who/who_model.py

src/flask_covid19/blueprints/data_who/who_service_update.py