Merge branch 'feat/publication_date' into 'develop'

src/api/serializers.py

src/backoffice/views/assemblies.py

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

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

src/core/migrations/0150_conference_registration_start.py

src/core/migrations/0151_conference_publication_date.py

src/core/models/base_managers.py

src/core/models/conference.py

src/core/tests/assemblies.py