Reworked spawn rates to reflect reality

In other words, how I thought the spawn controls work and how they actually work are practically opposites as was helpfully pointed out to me by Sophie Wright. Here's how it actually works:

The setting is a *1 in X* chance of the dungeon generating, not a percentage (which would make way more sense to use to me but whatever).

Previous settings were based on the assumption that spawns were percentages. The new numbers now reflect the intended spawn rates from the last build, also taking into account how well certain values worked previously.
Merge tag 'tags/v2.6.9.2' into serverExp