Convert all tables to SearchKit displays

This extension has 4 tables, which were rendered the traditional way with Smarty:

1. View all discount codes
2. View usage for a discount code
3. View usage for a contact (embedded on contact summary tab)
4. View usage for codes assigned to organization (embedded on organization contact summary tab)

This converts them all to SK displays and deletes the old page classes & templates.

Note: This will require a core version bump to 5.57