[scanner integration] Show moderate warning on directory entry page for landing pages with links to .onion addresses

[eloquence]

Part of epic #488. A landing page shouldn't like to an .onion address directly; attempts to click such a link in a clearnet browser open up an avenue for an adversary operating at the DNS level to gather data about potential SecureDrop users. In these cases, we should show the following warning (see #514 for overall warning logic and boilerplate text):

This landing page links directly to a Tor Hidden Service (.onion address). Any attempt to visit such a link in a regular browser will fail, but it may be detected by third parties.

[eloquence]

As of today, the landing page scanner logic is reporting no landing pages in the directory that have this problem (the only exception was https://freedom.press/people/kevin-poulsen/ , which has since been fixed).

[eloquence]

This appears to be working in current master (tested with https://eloquence.github.io/sdo-tests/onionlink.html since no live landing pages have this problem), but still uses placeholder text; please update consistent with the message above for now (we can always tweak further).