UUIDHelper improvements

[bigsir24]

Fixes UUIDHelper freezing your game when looking at signs or flags if the UUID was invalid or could not be fetched.

Apparently this was already fixed by #22, the rest should still apply

UUIDHelper#getNameI18nFromUUID should now be used for getting usernames if necessary, as it will return two different translated keys instead of null, based on why it couldn't get the username:

• None - The UUID was null or 0 (which is effectively invalid), signifies that a sign/flag has no owner
• Unknown - The UUID could not be matched to a username for various reasons
• Otherwise it returns the username for the given UUID

The UUID map should now also be updated when a player joins the server.

Misc:

• 03904e3 - Fixed bad isServer check crashing the game when switching from multiplayer to singleplayer