The persistentFlight option just kicks in flight whenever a player joins
the server mid-air. Servers with this will have the /fly command enabled
anyway, so it doesn't hurt.
When a player disconnects while flying in survival mode, they will
plummet to their deaths while reconnecting. These changes prevent that.
If a player is currently flying, their flight will not be disabled
unless the setGameMode method is called in the ServerPlayerEntity, which
is how the GameMode is usually set. Since the GameMode is initially set
using the interaction manager instead, players should be safe. :-)
