Restrict community creation to instance administrators

Problem

Currently, Fluxer only offers two states for community creation: either Single-community mode is enabled (the instance is locked to a single guild), or it is disabled (any registered user can create as many new communities as they want). For a gaming community, we need multiple distinct guilds to separate our different dedicated servers (DayZ, Garry's Mod, etc.) while keeping one main "Default" guild for general chat. Disabling Single-community mode allows any user to freely create servers, which consumes self-hosted resources and clutters the instance.

Proposal

Add a way to restrict guild/community creation to instance administrators or staff only. This could be implemented as a global toggle in the Admin Panel (e.g., "Allow standard users to create communities") or as a specific permission tied to user roles that can be removed for standard users. This would allow admins to provision the necessary spaces while keeping the infrastructure closed to user spam.

Notes

I've considered workarounds like hiding the "New Community" button with Custom CSS (not secure as the API endpoint remains exposed) or blocking POST requests at the reverse proxy level, but a native solution is much cleaner and easier to maintain for self-hosters. Keeping everything in a single guild and relying purely on hidden categories creates too much UI clutter and requires overly complex role management.