Support free tickets

[rohangpta]

in the view initiate_checkout, we exit early if cart total is 0.

Let's instead make it check individual tickets: all(t.price == 0 for t in cart.tickets.all()) and then simply "sell the tickets" to the person and then exit.

This also should be accompanied with a refactor. We have "sell the tickets" logic in complete_checkout. Move this to a separate helper function so that both the free ticket purchase path and paid ticket purchase path can use this shared function.

We also need one more refactor: the response returned should indicate that the tickets have been sold and the frontend doesn't need to call complete_checkout. This will require changing the response schema.

[aviupadhyayula]

Resolved in #658.