Revert "Merge branch 'feature/RegisterSpeaker' into 'develop'"

src/backoffice/forms.py

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

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

src/backoffice/templates/backoffice/assembly_event.html

src/backoffice/urls.py

src/backoffice/views/events.py

src/backoffice/views/users.py

src/core/migrations/0051_UniqueTogether2EventParticipant.py

src/core/models/events.py