Refactor Free vs Paid ticket purchasing

[charleslavon]

The current logic flow won't properly handle purchasing multiple ticket tiers or combining purchase of free/paid tickets. The user will potentially be redirected to Stripe before the free tickets are processed.

We should probably do something like the following:

1. Loop through all tickets to filter free and paid tickets before making any API requests.
2. Loop through all free tickets and make free purchase API requests for each.
3. Loop through all paid tickets to build out Stripe redirect
4. Notify user that their free tickets have been purchased/processed and that they'll be redirected to Stripe next to pay for remaining tickets.
5. Once Stripe purchase has succeeded, collect all secret keys for free and paid tickets to redirect user to purchased tickets route so they can view and download all tickets in a single group.

Originally posted by @calebjacob in #58 (comment)