Security module

We have a setup step to restrict console logins to users in the "admin" or "sudo" group. Currently this happens during freedombox-setup's setup script.

We want to automate the setup script to happen when freedombox-setup is installed, but the restricted console login can interfere with desktop display managers (or any non-sudo user). Debian users should be able to install freedombox-setup package without getting locked out from their systems.

This PR moves the configuration to happen during first boot setup, after the initial user is created. It also adds a "Security" page under System Configuration, with a checkbox to disable the restricted login.