Refactoring: rename covid19 to flask_covid19

src/flask_covid19/blueprints/data_owid/owid_test_views.py

src/flask_covid19/blueprints/data_owid/owid_views.py

...flask_covid19/blueprints/data_rki_cases/rki_test_views.py

src/flask_covid19/blueprints/data_rki_cases/rki_views.py

.../blueprints/data_rki_vaccination/rki_vaccination_views.py

src/flask_covid19/blueprints/data_who/who_test_views.py

src/flask_covid19/blueprints/data_who/who_views.py