Intuitive colours

[eyqs]

Make the design and colours more intuitive. The current colours are completely arbitrary and annoying to keep in mind what each colour means. Furthermore, some states are not particularly relevant to most users.

A possible solution is having three body colours for the state (green = taken, yellow = can be taken, red = cannot be taken), multiple border colours for more nuanced states (excluded, out of tree, etc.), and arrows for relationships between courses.