WIP: enable availability selection for AKs

AKModel/admin.py

AKModel/availability/__init__.py

AKModel/availability/forms.py

AKModel/availability/models.py

AKModel/availability/serializers.py

AKModel/locale/de_DE/LC_MESSAGES/django.po

AKPlan/locale/de_DE/LC_MESSAGES/django.po

...n/static/AKPlan/vendor/fullcalendar/bootstrap/LICENSE.txt

AKPlan/static/AKPlan/vendor/fullcalendar/bootstrap/README.md

AKPlan/static/AKPlan/vendor/fullcalendar/bootstrap/main.css

AKPlan/static/AKPlan/vendor/fullcalendar/bootstrap/main.d.ts

...n/static/AKPlan/vendor/fullcalendar/bootstrap/main.esm.js

AKPlan/static/AKPlan/vendor/fullcalendar/bootstrap/main.js

.../static/AKPlan/vendor/fullcalendar/bootstrap/main.min.css

...n/static/AKPlan/vendor/fullcalendar/bootstrap/main.min.js

.../static/AKPlan/vendor/fullcalendar/bootstrap/package.json

AKPlan/static/AKPlan/vendor/fullcalendar/core/LICENSE.txt

AKPlan/static/AKPlan/vendor/fullcalendar/core/README.md

AKPlan/static/AKPlan/vendor/fullcalendar/core/locales-all.js

...static/AKPlan/vendor/fullcalendar/core/locales-all.min.js