RM generation: improve metal mines generation
Introduces a new helper to place mines. use it on 'Best for MP' maps for stone and metal.
The amount of metal will scale with the # of players, and will place slightly more mines overall for fairer distribution.
This should make 1v1 and 4v4 more playable on these maps.
Differential Revision: https://code.wildfiregames.com/D4148