Make lobby signs optional

[Sowgro]

I do not use the lobby signs feature, and keeping track of them is annoying when I switch out my lobby world. This pull request makes them optional, and tweaks the arena validation system accordingly.

While I was at it I made a few other small fixes:

• The kit help message will no longer show up if there are no kits.
• Changed Random to ThreadLocalRandom for compatibility with Java 16.