Specify how we want to sync packages from Debian

More specifically: which packages? when? into which APT suite(s)? Think about it, come up with a proposal, discuss this with the community, lead this discussion to a conclusion.

Blueprint: https://tails.boum.org/blueprint/freezable_APT_repository/

Parent Task: #9487 (closed)

Original created by @intrigeri on 9488 (Redmine)