Save pending game submissions in database

Currently they're sent to a fixed address in JSON format and encoded to a JWT, which is then decoded when someone clicks that link. Apart from URL length being limited size, this is kinda janky.

Instead, save the pending/unconfirmed/drafted/unpublished games in the DB - possibly even the normal games DB, but with some kind of flag. Admins can see all games, including draft ones (so that they can verify they look good on the page before publishing them). For now it's probably not necessary to add much additional admin UI, as it's easy enough doing tweaks in the DB directly.