New endpoint: verify ticket validity

[oddstr13]

Create API endpoint for checking that a ticket is valid (ticket id is separate from nfc armband id).
In the final implementation, this should check an external API (the ticketing system where people actually buy the tickets).
This API endpoint should take a string (ticket), and return a boolean. For now, return True when the ticket is exactly test123, and False otherwise.
Add a TODO: Implement ticket check comment to the view, so that we remember to implement proper functionality in the future.