Now that we use pytest-asyncio, it is easier to test the dead links in an async context and not need to worry about weird issues with the aiohttp client session being used outside of an async context. The places where the function was being called before (only one, in search) can now call `sync_check_dead_links` instead without any change to the overall behaviour of the app. It just makes the tests easier to configure now that we actually have real async contexts going on in some places.

It is worth reverting the httpx addition back to the aiohttp version. Use the same implementation of `get_aiohttp_session` but actually use `aiohttp.ClientSession` instead. Pass `session.close` instead of `aclose` to the application shutdown callback. Everything else is basically the same. You will see the issues I have noted in the PR comment of the event loop closing if you replace the `web` service command in docker-compose with `gunicorn --reload`. The rest of the relevant configuration will get picked up from the gunicorn configuration file.