Add support for the -updates and -security stable repositories

The new release flow states that stable releases need two extra APT repositories to be configured by default:

• $RELEASE-security (e.g. v2020-updates) for urgent security fixes, all products should have this enabled
• $RELEASE-updates (e.g. v2020-updates) for updates to be included in the next point release, usful for testing, actual products should wait for the updates to be published as part of the point release

Automatically set up the security and updates repository if the current release is not a development nor a preview one.

This forward-ports some changes (1c9cda15 and c5d5450d, plus !222 (merged)) from the apertis/v2019 branch which for some reason were left behind.