Refactor ConferenceSelectionView

src/backoffice/forms.py

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

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

src/backoffice/templates/backoffice/base.html

...ackoffice/templates/backoffice/conferences/selection.html

src/backoffice/urls.py

src/backoffice/views/conferences.py

src/backoffice/views/misc.py

src/backoffice/views/mixins.py