[scanner integration] Associate source interface scan results with directory entries

[eloquence]

As part of #488, we are interested in showing some basic information about the source interface on the directory page:

• the current version of SecureDrop
• an "instance down" indicator if the instance is down
• an "estimated downtime" indicator if the instance is down

Note that this is very minimal information. We have no need for extensive per-host histories on SecureDrop.org, nor are we currently planning to integrate alerting directly with SecureDrop.org. Nagios/Icinga style monitoring/alerting should be considered a separate concern, which may use the same scanner (https://github.com/freedomofpress/infrastructure/issues/1084) and indeed be driven by the same scanner runs.

Scan objects should contain timestamp, version number, onion URL, public key fingerprint, and other scan information that the scanner returns.

Given that our concerns here are very basic, I would propose only two scan objects be associated with each directory entry:

• most recent scan result
• most recent successful scan result

This should be sufficient to meet the functional requirements. (To mitigate against cases where our scanner is falling over, we can avoid reporting inaccurate downtime by not displaying a scan result at all if the most recent scan is insufficiently recent.)

This task has two main parts, which we may wish to split up further depending on implementation strategy:

• extending the directory entry model to allow for associating this data with an entry
• integrating the actual scan results (e.g., by enabling SecureDrop.org to consume data from Nagios scan runs).