Disable contact forms on past WordCamp sites

[dd32]

Summary

• Replaces Jetpack contact form HTML with a message on past WordCamp sites to reduce spam to organizers
• Forms are disabled when either:
  1. A newer event site exists for the same city (using existing get_latest_home_url() logic), OR
  2. The event ended more than 18 months ago (covers cities that stopped hosting events)
• The replacement message directs visitors to the latest edition (if one exists) and always includes a link to report Code of Conduct concerns to the WordPress Incident Response Team at reports@wordpress.org
• Uses the jetpack_contact_form_html filter as suggested by @jeherve in the issue discussion
• Added to latest-site-hints.php as suggested by @dd32

Implementation details

• Reuses the existing get_latest_home_url() function and get_wordcamp_post() end date logic
• Events still in progress (within 1 day of end date) keep their forms active
• The 18-month grace period ensures forms stay active for slightly longer than a year after the event, giving time for a new edition to be scheduled

Test plan

• Visit a past WordCamp site that has a newer edition — verify contact form is replaced with message linking to the new site
• Visit the most recent WordCamp site for a city — verify contact form still works
• Verify the Code of Conduct reporting link appears in the replacement message
• Visit a site whose event ended less than 18 months ago with no newer site — verify form still works
• Visit a site whose event ended more than 18 months ago with no newer site — verify form is replaced

Fixes #1468

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