Refactoring: rename covid19 to flask_covid19

src/flask_covid19/blueprints/data_divi/divi_model.py

src/flask_covid19/blueprints/data_ecdc/ecdc_model.py

src/flask_covid19/blueprints/data_ecdc/ecdc_model_import.py

src/flask_covid19/blueprints/data_owid/owid_model.py

src/flask_covid19/blueprints/data_owid/owid_model_import.py

src/flask_covid19/blueprints/data_rki_cases/rki_model.py

...ask_covid19/blueprints/data_rki_cases/rki_model_import.py

.../blueprints/data_rki_vaccination/rki_vaccination_model.py

...ints/data_rki_vaccination/rki_vaccination_model_import.py

src/flask_covid19/blueprints/data_who/who_model.py

src/flask_covid19/blueprints/data_who/who_model_import.py