Add repeatable Venue Name field for multi-venue events

[dd32]

Summary

• Adds a new multi-text field type to the event tracker, replacing the single text input for Venue Name
• Allows Campus Connect and other events to list multiple venues with add/remove buttons
• Backwards compatible: existing single-string venue names are automatically displayed as a single entry
• Template functions join multiple venues with commas for frontend display

Screenshots

Before: Single text input

After: Repeatable text inputs with add/remove buttons

Test plan

• Edit a WordCamp or Campus Connect event in wp-admin
• Verify the Venue Name field shows an input with a remove (x) button and an "+ Add another" button
• Click "+ Add another" to add a second venue, enter a name
• Save the event and verify both venues are preserved
• Remove a venue and save, verify it is deleted
• Verify frontend display joins venues with commas

Alternate approach to #43 per review feedback.
Fixes WordPress#1604

Generated with Claude Code