admin: Store items for event

[katajakasa]

Add a store items view to admin. It should show all items that can be sold for selected event.

• Add a view for adding/editing/deleting/viewing store items
• Show how many items have been sold and what is the total
• Do not allow deletion if there are any sold items!
• Add vitest unit-tests
• Add APIv2 entrypoints for managing the items
• Add APIv2 unit-tests
• Check for store items permission!

[katajakasa]

Done