Random scenario selection options

Add two options in the World Defaults menu to enable random scenario selection for random characters.

Options:
- Random character and scenario  
- No challenges

This PR is a follow up to https://github.com/CleverRaven/Cataclysm-DDA/pull/9696.
I'd rather avoid telling folks that two keys both do the same thing if they don't, and here one rerolls more than the other. I'd predict bug reports if I merged that string as-is, sorry.