hangar creation support (visual part of #89)

src/backoffice/forms.py

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

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

src/backoffice/templates/backoffice/assembly_room.html

src/backoffice/templates/backoffice/room-new.html

src/backoffice/views/assemblies.py

src/core/integrations/__init__.py

src/core/integrations/rc3hangar.py

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

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

src/core/migrations/0001_initial.py

src/core/models/rooms.py

src/rc3platform/settings/base.py