Add product purchases relation manager to user admin page

[simonhamp]

Summary

• Adds a Products relation manager tab on the user edit page showing all products a user has purchased or been comped
• Supports searching, sorting, filtering by comped status, creating comped licenses, and deleting licenses
• Adds a migration for the is_comped column on product_licenses (existed in DB but had no migration)
• Updates the sales_view to reference the actual is_comped column instead of hardcoding 0
• Adds is_comped boolean cast to the ProductLicense model

Test plan

• 6 new tests covering listing, user scoping, comped status, create, delete, and filtering
• Verify the Products tab appears on the user edit page in the admin
• Verify creating a comped product license works
• Verify the sales view still works correctly after the migration change

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