Clean up some deprecation warnings

[rtibbles]

Summary

The migration to Django 3.2 left some deprecation warnings, this cleans up the following:

• We no longer need to define the default app config in the app's init.py, Django automagically discovers it in the apps.py
• The DRF filter_fields has been deprecated in favour of filterset_fields
• Django will soon disallow objects that are not compatible with copy.deepcopy() from being set on the class object during setUpTestData - the workaround for this is to do it in setUpClass. If needed in setUpTestData as it is here, this is done prior to the super call (during which setUpTestData is called).
• DRF has deprecated the get_filter_class method in favour of get_filterset_class method
• SQLAlchemy has deprecated a large range of its API, especially around its ORM, so I have made numerous updates to our use of SQLAlchemy here, in line with https://docs.sqlalchemy.org/en/20/changelog/migration_20.html#migration-to-2-0-step-two-turn-on-removedin20warnings
• Removed our usage of most bare assert statements outside of tests, except where it was very clearly intended to be a developer only warning

References

There is no open issue for this, I just got sick of seeing these warnings when running tests locally.

Reviewer guidance

If tests all pass, this should give some decent confidence - but I am targeting this to a patch release to defer the risk slightly.

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.17.0b3.dev0+git.25.g3b58fe1d-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0b3.dev0+git.25.g3b58fe1d-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0b3.dev0+git.25.g3b58fe1d.dmg
Android Package (APK)	kolibri-0.17.0b3.dev0+git.25.g3b58fe1d-0.1.3-debug.apk
TAR file	kolibri-0.17.0b3.dev0+git.25.g3b58fe1d.tar.gz
WHL file	kolibri-0.17.0b3.dev0+git.25.g3b58fe1d-py2.py3-none-any.whl

[bjester]

Changes make sense and look good to me