QR code refactor

[damienlaine]

Allow creating custom aliases in advance for QR code generation, even before the associated session exists.

Typical use case:
Generate and print QR codes ahead of time (e.g., on flyers or posters) using a simplified URL alias, which can later be associated with the actual technical session URL.

[damienlaine]

Maybe we can also use something like :
https://github.com/738/awesome-url-shortener

Provided they offer an API