Generate QR paid code

[VladimirPokorny]

• add method to the app which will create the QR pay code
• it will save QR code as a png in format by the QR pay standard: https://qr-platba.cz/
• the new model field should be created in Ministrant model - qr_pay_code
• the qr pay codes should be exported into media/qr_codes folder