[scanner integration] Display "Unreachable" icon and SecureDrop downtime

[eloquence]

Part of epic #488. If the source interface is down (#501), we should display the "Unreachable" icon from #489 and the estimated downtime on both directory entries and the directory page, provided we have a sufficiently fresh scanner data.

Proposed display format:

• ["Unreachable" icon] SecureDrop unreachable. Estimated downtime: 4 hours

A more compact display may be appropriate for the directory view.

[eloquence]

Discussion: Given the nightly reboot and occasional Tor network issues, should there be a minimum threshold before we report downtime? My view is that a threshold makes sense (perhaps ~30 minutes), and that it may be ideal to pick the same threshold for external alerting, as well.