Use thumbnails for avatars and covers to reduce bandwidth


Image thumbnails [Manual action required]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To reduce bandwidth usage on slow or limited connexions and improve performance
in general, we now use smaller images in the front-end. For instance, if you have
an album cover with a 1000x1000 pixel size, we will create smaller
versions of this image (50x50, 200x200, 400x400) and reference those resized version
when we don't actually need the original image.

Thumbnail will be created automatically for new objects, however, you have
to launch a manual command to deal with existing ones.

On docker setups::

    docker-compose run --rm api python manage.py script create_image_variations --no-input

On non-docker setups::

    python manage.py script create_image_variations --no-input

This should be quite fast but may take up to a few minutes depending on the number
of albums you have in database. It is safe to interrupt the process or rerun it multiple times.