chore(schedule): add version field on confernce and schedule model

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

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

...ore/migrations/0166_schedulesource_last_import_version.py

src/core/migrations/0167_conference_schedule_version.py

src/core/models/conference.py

src/core/models/schedules.py