SCRUM#230: add job on .gitlab-ci.yml to deploy package to stable repository.

See merge request !3

SCRUM#212: Completely rewrite le-autoupgrade

See merge request !2

We are no longer implementing a customized version of unattended-upgrades.
Instead, we'll just use unattended-upgrades do set up the desired behavior.

Issue SCRUM#159: Fix in configuration to get updates from the right repositories

See merge request !1