Merge branch 'feature/message_restrictions' into 'develop'

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

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

src/core/migrations/0055_dm_restrictions.py

src/core/models/conference.py

src/core/models/users.py

src/plainui/forms.py

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

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

src/plainui/tests.py

src/plainui/utils.py

src/plainui/views.py

src/rc3platform/settings/base.py