Allows updating the credentials of the announce session, thus allowing credentials changes to be reflected before citra restart. To avoid race conditions and web errors (you can only update the room that you created, i.e. changing credentials halfway will make it break), now you can only use the Citra Web Services settings when not hosting a public room.
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".
Since C++17, the introduction of deduction guides for locking facilities
means that we no longer need to hardcode the mutex type into the locks
themselves, making it easier to switch mutex types, should it ever be
necessary in the future.
