Event: add banner_image_url and import it in .load_data()

src/core/admin.py

src/core/locale/de/LC_MESSAGES/django.po

src/core/locale/en/LC_MESSAGES/django.po

...s/0159_event_banner_image_url_alter_event_banner_image.py

src/core/models/events.py

src/core/models/users.py

src/hub/settings/base.py