Landing page scanner should ignore entries that aren't live

[eloquence]

https://securedrop.org/admin/scanresult/ includes a lot of recent scans for entries that are no longer live. There's no benefit in these scans and many of them will fail due to dead URLs. The scanner should only scan entries that are currently live.

[chigby]

Timebox this to ~1 hour

Here's what I came up with: develop...default-scan-ignore-nonlive-entries