Upgrade to Django 5.2

[ffont]

We should upgrade to the next LTS version of Django (5.2)

Steps:

• [ ] Clear Django 5 warnings (run tests with warnings option enabled to see them). This will require at least upgrading SESSION_SERIALIZER to django.contrib.sessions.serializers.JSONSerializer, which causes problems because we sometimes serialize Sound and File objects in the session. We need to reimplement the way we do that (or extend the serializer so it can serialize/unserialize these data types?)

• [ ] Make sure Django dependencies are fine:

• dj-database-url > should be fine with 5.x
• django-admin-sortable > unsure about 5.x. We could look at django-admin-sortable2 ifneedbe.
• django-amazon-ses > unsure about 5.x
• django-cors-headers > should be fine with 5.x
• django-debug-toolbar > should be fine with 5.x
• django-extensions > should be fine with 5.x
• django-multiupload > should be fine with 5.x
• django-oauth-toolkit > should be fine with 5.x
• django-object-actions > no notes about django compatibility, but latest version works with 4.2
• django-ratelimit > no notes about django compatibility, but latest version works with 4.2
• django-recaptcha > unsure about 5.x
• django-redis > unsure about 5.x
• django-silk > should be fine with 5.x
• djangorestframework > should be fine with 5.x
• sentry-sdk[django] > should be fine with 5.x

• [ ] Update default checkbox render behaviour so that checkbox INPUT elements are rendered inside LABEL elements. Then we can remove the ugly hack in checkbox.js which moves checkbox elements inside labels. In Django 5.x, this change should be easily achieved, see this: https://docs.djangoproject.com/en/5.1/topics/forms/#reusable-field-group-templates

Potential follow-up steps:

• ~~replace django-redis with django.core.cache.backends.redis.RedisCache? See https://github.com/jazzband/django-redis/issues/585 for a discussion~~
• ~~Switch to Psycopg 3 https://docs.djangoproject.com/en/4.2/releases/4.2/#psycopg-3-support~~

[alastair]

I vote that we remove django-redis. We only use it for basic django caching, and so therefore using the built-in caching backend will be fine