Player session management with session code generation

[thaleius]

• generating new player session with session code upon accessing player page
• code consists of four digits A-Z0-9
• integrated foundation to persistently store sessions
  • up until now no solution found to re-identify client endpoint after backend restart because of client endpoint id change on backend restart
• integration with qr code generation on player and client page using url scheme {url.origin}/client/{code}
• frontend entrypoint: /client/([A-Z0-9]{4}), where ([A-Z0-9]{4}) resembles the four digit code passed by URI path
• restyling of the qr code elements so that it does not overflow the page