This adds `#nullable disable` to all existing code (except where null is impossible like enum files), so it can be migrated incrementally.

MongoDB support unnecessarily complicated the code and there's no need to run distributed servers in the foreseeable future. This keeps the abstract storage interface so we can wrap a distributed cache in the future.

This commit moves the core update-check logic serverside, and adds support for community-defined version mappings. For example, that means false update alerts can now be solved by the community for all players.