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.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>