Frontend: Support multiple distinct track colors

[tuffnerdstuff]

It would be nice to have a configurable set of distinct colors, one for each new user/participant of a group share. Currently all the tracks are rendered using the same color (trail_color), which gets confusing if you have bunched/crossing tracks. Therefore I would suggest the following changes:

• rename trail_color to trail_colors and make it an array
• choose 16 distinct colors as default value for trail_colors
• for each new user choose the color at the next index in trail_colors
• if more users joint than there are colors, just wrap around

[rustyduffer]

This would be great :)