Add industry gatherings page and external event support

[bokelley]

Summary

• Create dedicated /industry-gatherings page with API endpoint for browsing industry event attendee groups
• Update nav to link to /industry-gatherings instead of /committees?type=industry_gathering filter
• Fix event timezone display - use event's timezone instead of viewer's local timezone
• Add external event support for third-party events (CES, Cannes Lions, etc.)

Changes

Industry Gatherings Page

• New /industry-gatherings route and HTML page
• New /api/working-groups/industry-gatherings endpoint that returns gatherings with linked event info
• Separates upcoming vs past gatherings
• Shows event banner with date, location, and image
• Links to Slack channels and event detail pages

Event Timezone Fix

• All event datetime displays now use the event's stored timezone instead of the viewer's local timezone
• Affects: events.html, event-detail.html, admin-events.html, working-groups/manage.html, working-groups/detail.html, industry-gatherings.html

External Event Support

• New external_registration_url and is_external_event fields in events table
• Committee admins can mark events as third-party when creating/editing
• Event detail page shows "Register on Event Site" button for external events
• "Industry Event" badge displayed for external events

Test plan

• Navigate to /industry-gatherings - page should load with any existing industry gatherings
• Click Events → Industry Gatherings in nav - should link to new page (not committees)
• Create an event with PT timezone, view on events page - should display in PT not local timezone
• Create an external event via working group manage page - should show external registration URL field when checkbox is checked
• View external event detail page - should show "Register on Event Site" button and "Industry Event" badge

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