Decreased Default Gateway Generation Chance

Decreased the default chance of a gateway generating. This won't affect
users unless they clear their config files. The decision is based on
user feedback and recent experiments on how common gateways could be.
Dungeon Selection Fix and Chain Config Change

Changed how Random is initialized in SchematicLoader to prevent issues
with dim doors in the same chunks leading to the same dungeon. The
selection seems much more varied now. Also changed the hardcoded config
for the default dungeon chains to the one we'll be using later (from a
file). For testing purposes.