Scan for links to securedrop.org from landing page

[eloquence]

We recommend not linking directly to SecureDrop.org (https://docs.securedrop.org/en/stable/deployment/landing_page.html#avoid-direct-links-to-securedrop-org) but instead pointing to the .onion service, so the scanner should check for whether such links exist in the HTML.

Not part of epic #488 since its less valuable to the user (they're already on SecureDrop.org) and mainly information we may wish to capture for verification and outreach purposes.