Shopping cart

[kubapoke]

This pull request contains implementation of the shopping cart for buying new tickets. This is a huge PR, so I'll give a brief description of what it contains and what remains to be added.
The PR contains:

• Implementation of the shopping cart class for buying tickets.
• The full functionality of the cart, supported by API endpoints (adding tickets to cart, removing them, retrieving items in cart, calculating due amount, and finally - checking out).
• Adding new tickets to the database via checkout.
• A dynamic reservation counter maintained in Redis - counts tickets in carts and is synced manually every few minutes by a background service.

Note that for the functionality to work you need to connect to both Redis and the Payment gateway. One new migration has been also added.

The functionality that needs to be added to the shopping cart:

• Buying tickets resold by other users.
• Implement the ability to set seats/name on ticket parameters for new tickets.
• The unit tests are not present for now, I will try to add them ASAP.

[kTrzcinskii]

Great work 🥳 and thanks for fixing ToObjectResult