[scanner integration] Automatically delist instances with major landing page HTTPS issues

[eloquence]

As part of implementing #488 and once we are able to delist instances (#491), we should automatically do so if the landing page scanner detects any of the following HTTPS issues:

• no HTTPS on landing page
• mixed-content (HTTP + HTTPS) on landing page

Serving landing pages over unencrypted or improperly encrypted connections is a big "no" that compromises source security, and may indicate other configuration problems further down the stack. Until such issues are fixed, an instance should not be listed in the directory.

Automatic delisting must trigger some form of alert (email or Slack).