mint-arena: Add more player effect colors

color1/color2 cvars can now be set to 8 to 13 for new colors.

In the UI, if the artwork piece for a new color is missing,
the white artwork is used and colorized.

Backported from Turtle Arena.
mint-arena: Choose bot effect color defaults using their name

All bots using the same rail color is kind of dull. Pick colors using bot's name.