Specify how we want to sync packages from Debian

Originally created by @intrigeri on #9488 (Redmine)

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)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information