Packaging securedrop export code

[redshiftzero]

We currently have logic in the send-to-usb script for exporting files to USB drives. We'll be adding more export logic as time goes on e.g. as we add support for printing and (eventually) other export options. We should consider how we'll be updating this logic: we can put files in place via salt in dom0, but a more robust approach would be taking the approach we use in the sd-proxy and sd-svs VMs, which is packaging up the code (in the securedrop-proxy and securedrop-client debian packages) and then just using salt to ensure that package is installed.

[emkll]

Since there are a lot of moving parts, tracking progress/status in this comment

• Adds modular version of Qubes Workstation send-to-usb script securedrop-export#1 create securedrop-export repo and merge code into there
• Adds securedrop-export build logic securedrop-builder#69: add packaging logic to securedrop-debian-packaging
• Add makefile and ci for tests securedrop-export#2: Add tests in CI + update_version.sh to support automatic packages
• Add nightly builds for securedrop-export package securedrop-builder#71: Provide CD for securedrop-export packages
• Packages uploaded to apt-test-qubes.freedom.press (if the above is merged, will be done automatically by daily cron)
• Remove salt provisioning for sd-export files #298 : securedrop-workstation remove of export logic and install securedrop-export package