APT snapshots: add Bullseye security & backports

Beware, the layout for the security repo has changed and I don't know how tails-prepare-tagged-apt-snapshot-import will cope with it.

• Debian Bullseye security
• Debian Bullseye backports
• Tor Project Bullseye
• Check how tails-prepare-tagged-apt-snapshot-import copes with the new layout of the security repo, adjust as needed.