security: Integrate debsecan

debsecan is a utility that lists security vulnerabilities for installed packages.

Example: debsecan --format packages | uniq -c to show the count for each installed package.

There are several ways this information could be useful:

1. Vulnerabilities in freedombox package itself.
2. Vulnerabilities in packages installed by an app.
3. Vulnerabilities in other parts of FreedomBox system.

This is especially useful for packages installed from backports, as backports does not have the same level of security support (https://backports.debian.org/FAQ/).