The `Register()` function can now handle error results and the error will be passed immediately to the Qt frontend, instead of being ignored silently and failing later with a "Room is not registered".
* Change authentication system to JWT
* Address review comments
* Get rid of global variable, fix some documentations, fix a bug when verificating
* Refactor PostJson to avoid code duplication
* Rename jwt_token, add functionality to request a new JWT when getting a 401
* Take bools by value instead of const reference
* Send request again when JWT is invalid and use forward declarations
* Omit brackets
