[scanner integration] Scan for direct .onion address links on landing page

[eloquence]

Part of epic #488. We recommend not to directly link .onion addresses (https://docs.securedrop.org/en/stable/deployment/landing_page.html#do-not-hyperlink-onion-addresses), so we should verify whether such links are present and record it in the scan results.

While this only impacts users who click on the link in a non-Tor browser, in those cases, it does lead to a DNS leak of their attempt to access SecureDrop, so it warrants a "moderate" warning

[eloquence]

This is actually already supported in the current scanner code (see validate_onion_address_not_in_href in scanner.py), it's just not used for the grades. Closing.