Image App: Cropping feature
from #174 define ratios/dimensions for thumbnail / frontend usage / ...
implement views and data storage
see e.g.
- https://pypi.org/project/django-image-cropping/ (uses JS: https://jcrop.com/)
- JS: https://fengyuanchen.github.io/cropperjs/ via https://simpleisbetterthancomplex.com/tutorial/2017/03/02/how-to-crop-images-in-a-django-application.html
- https://github.com/ninapavlich/django-imagekit-cropper (very alpha stage acc to docs) works with https://github.com/matthewwithanm/django-imagekit (deb pkg exists) uses Python Pillow (fork of PIL)