Move application configuration logic away from Ansible and into the .deb package

[emkll]

Feature request

Description

Ansible task is currently generating values to initialize/provision/configure the SecureDrop application (see https://github.com/freedomofpress/securedrop/blob/develop/install_files/ansible-base/roles/app/tasks/initialize_securedrop_app.yml )

SecureDrop app package should configure SecureDrop app-specific values (eg: configuring keys/pepper values, initializing database

User Stories

As a package maintainer, I do not want to edit infrastructure code to modify the configuration of the application.
As an infrastructure developer/admin, I would like the package manager/application to automate it's configuration.

[eloquence]

This is still a worthwhile improvement and consistent with our goal to reduce Ansible footprint. Good candidate for mid-term improvements to the provisioning story or a good ticket for a volunteer familiar with Debian packaging & Ansible.