[scanner integration] Display SecureDrop version on directory entry

[eloquence]

As part of #488, we want to display the version a SecureDrop instance is currently running. Per #501, if we have a sufficiently recent scan (<1 week old), we should display this information as in the following example:

• SecureDrop version: 0.7.0 (as of 5 minutes ago)

For now, we do not need to worry about displaying version number related warnings to the user. We should automatically delist instances that get stuck on an older version, which is a separate issue.