Update to Django 3.2

Django 3.2 has been released, and we should update to it. Doing so, the following changes in Django seem relevant:

• Automatic AppConfig discovery — we can drop the default_app_config from all __init__.pys
• DEFAULT_AUTO_FIELD is recommended to be BigAutoField, even though it is not the default yet. We should consider setting it explicitly (but need to check whether this will cause migrations in libraries if set…)
• Functional indexes are now supported — we need to check models thatare often queried using a database function (e.g. aggregates), and add functional indexes for those queries (but leave the index out if write performance is more critical than search performance!)
• makemigrations can now be called without a database connection — that means we can add a check for missing migrations to the pipelines
• Index.include and UniqueConstraint.include are now supported for creating covering indexes — we should check whether we have queries that can be turned into index-only scans that way
• FilteredRelation now supports nested relations (probably most important for Alsijil)
• QuerySet.alias can be used instead of QuerySet.annotate for aliases that are only needed for filtering (probably most interesting for Alsijil)
• Transforms in expressions — not sure
• SECRET_KEY is not needed anymore for running most management commands — we can therefore remove the secret key from at least the migration job container in K8s