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

docker/alpine.Dockerfile

@@ -31,7 +31,8 @@ RUN pip install -U pip && pip install -r requirements.txt -r requirements.dev.tx
 COPY . .
 COPY --from=node-build /etc/linkding .
 # run Django part of the build
-RUN python manage.py compilescss && \
+RUN mkdir data && \
+    python manage.py compilescss && \
     python manage.py collectstatic --ignore=*.scss && \
     python manage.py compilescss --delete-files