Refactor rank check: room only ranks do not interact with demote/promote
The function getNextGroupSymbol now takes an extra parameter to exclude room only ranks, which are defined in config.
The function will try to find the closest non room only rank with a safeguard against maliciously or badly crafted config.js to crash the server.
If no rank is found regardless of room only or not status, the first or last rank are returned accordingly.
This refactor allows getNextGroupSymbol's name to keep true to its functionality, while also providing a way to exclude crafted ranks for rooms or otherwise from the global demote/promote scheme.