Merge branch 'fix/assembly-slugs' into 'develop'

src/api/serializers.py

src/api/views/badges.py

src/api/views/mixins.py

src/backoffice/forms/__init__.py

src/backoffice/forms/assemblies.py

src/backoffice/views/moderation/mixins.py

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

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

src/core/migrations/0170_alter_assembly_slug.py

src/core/models/assemblies.py

src/core/validators.py

src/plainui/urls.py

src/plainui/views/assemblies.py