Add graffiti to parametrized wall palette

[Milopetilo]

Summary

Content "Walls now have a chance of having graffiti on them"

Purpose of change

Was lurking about the discord recently and saw someone bring up the fact that only vandalized buildings have graffiti on them even though there were riots and people generally behaving unwise weeks prior to game start. This makes it so that there sometimes is graffiti on all walls.

Describe the solution

Add nests to parametrized_walls_palette to have graffiti spawn, I've chosen to copy over the nests from standard_domestic_damage_large_palette which has a chance of 1/300 per wall tile, which comes down to ~1 in 3 buildings to have a piece of graffiti on them. This may be too low but I'd rather have too little than too much.

Describe alternatives you've considered

Make a new palette for ungraffitied walls and make the normal walls include graffiti, is still very easily possible if needed.

Testing

Game loads, walls have graffiti every now and then.

Additional context

I've elected to not touch inner walls since that will lead to graffiti being inside buildings that are fully locked sometimes. I've not added it to the highrise walls palette because those walls are high and usually unreachable.