Replace django-background-tasks with huey (#657)

* Replace django-background-tasks with huey

* Add command for migrating tasks

* Add custom admin view

* fix dockerfile

* fix tests

* fix tests in CI

* fix task tests

* implement retries

* improve config

* workaround to avoid running singlefile tasks in parallel

* properly kill single-file sub-processes

* use period task for HTML snapshots

* clear task lock when starting task consumer

* remove obsolete cleanup task command

bootstrap.sh

@@ -18,6 +18,8 @@ python manage.py migrate
 python manage.py enable_wal
 # Create initial superuser if defined in options / environment variables
 python manage.py create_initial_superuser
+# Migrate legacy background tasks to Huey
+python manage.py migrate_tasks
 
 # Ensure the DB folder is owned by the right user
 chown -R www-data: /etc/linkding/data