Tails based on Debian Stretch

This ticket is only here so that we can mark this migration as blocking other tickets.

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

Related issues

• Blocked by #10298 (closed)
• Blocks #11293
• Blocks #10234
• Blocks #11636
• Blocks #11912 (closed)
• Blocks #8173 (closed)
• Blocks #12605 (closed)

Original created by @intrigeri on 10289 (Redmine)