map: add POIs and layers

src/api/urls.py

src/api/views/maps.py

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

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

src/backoffice/templates/backoffice/base.html

src/backoffice/templates/backoffice/map_layer_form.html

src/backoffice/templates/backoffice/map_layer_list.html

src/backoffice/templates/backoffice/map_poi_form.html

src/backoffice/templates/backoffice/map_poi_list.html

src/backoffice/urls.py

src/backoffice/views/map.py

src/backoffice/views/mixins.py

src/core/admin.py

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

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

src/core/migrations/0118_map.py

src/core/models/__init__.py

src/core/models/assemblies.py

src/core/models/conference.py

src/core/models/map.py