Nagios replacement: Scan source interfaces through Tor

[redshiftzero]

We should scan source interfaces and automatically send emails to admins upon multiple failed scans (we'll still want to retry the scan several times before sending an email due to Tor flakiness).

User Stories

As a SecureDrop administrator, I want to be alerted if my SecureDrop source interface is down.

[redshiftzero]

Note: https://stem.torproject.org/ is the right tool for the job here

[redshiftzero]

Here is a link to the GSoC project about this ticket.

[eloquence]

The GSoC project never came through and we have since undertaken new work related to this, such as the efforts documented in https://github.com/freedomofpress/infrastructure/issues/1084 . I would argue that we get the biggest bang for the buck out of the integration tasks identified in #488 and potentially migrating to a Nagios alternative for alerting.

Nagios, for all its flaws, has features like per-host histories, warning levels, annotations, etc.; better options exist. However, leaving this open for discussion.