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

With this change, only the main version has a direct download.

Showing beta info here caused a few issues:
* The vast majority of players don't use the game beta, so they were often confused about which version to download.
* Beta versions typically have much longer release info (e.g. detailed summary, release notes, caveats and warnings, etc), and the extra download button made the player guide button under it less prominent and visible. Those both contributed to information overload and the above confusion.
* Unlike main versions, beta versions aren't permanently archived on GitHub (since the beta branch is routinely rebased onto the latest stable update). That makes it messy to manage beta releases through GitHub.

Instead there's now a message under the download button which clearly links to where the beta version can be downloaded.

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.