provide reusable script for rendering templates from QubesDB variables

[cfm]

Description

securedrop-workstation should provide a reusable script that VMs can use to template configuration files from QubesDB for use by third-party programs.

• Prototype: https://github.com/freedomofpress/securedrop-client/blob/27cc69d7a87c937ff31bae9190b68ee2138525b8/with-qubesdb

How will this impact SecureDrop/SecureDrop Workstation users?

No user-facing changes.

How would this affect the SecureDrop Workstation threat model?

Discussed in #936 and #1013.

User Stories

• refactor service configuration from Salt (at provisioning) to systemd (at boot) #1004
• Set logvm via vm-config #1029