Build sample distro-tracker database ready to use for new contributors

Currently the initial ./manage.py tracker_update_repositories takes very long (see https://bugs.debian.org/755043) and can be a showstopper for some contributors. It would be great if we could save them this pain by providing them a ready to use SQLite test database with relatively recent data.

Ideally this can be done as part of GitLab CI (with a scheduled job)... but we probably need to find solutions to avoid hitting a time limit for jobs. And possibly also to reduce the size of the initial database (reduce the import to a subset of packages?).

Resources:

• See https://docs.gitlab.com/ee/user/project/pipelines/job_artifacts.html#downloading-the-latest-artifacts to find the stable URL of the latest artifact.