[0.12.0] Validate Tor version at install time

[emkll]

Description

If the apt server used for a SecureDrop install (e.g., apt.freedom.press) does not serve a tor package, the install logic will install the Tor package from Ubuntu upstream, which is quite old (0.2.4.27). It is the only package that mirrored on apt.freedom.press that also exists in upstream Ubuntu apt repos.

We should implement a verification in the install logic that ensures the version of Tor at install time is the one provided by FPF (and not the upstream-provided version).

I have added it to the pre-flight checklist for 0.12.0: #4124