Graphs have customizable line colors and integrated legend

See https://github.com/concord-consortium/lab-grapher/pull/9
Graphs not only have customized line colors, but those colors are used as part the generated legend!
That's *two* merge requests for the price of *one*! :+1:

Also did some minor formatting and code cleanup.
[#74535922]
[#74536668]
Right, and right again. I agree with the HTML color definitions, but that may break some added transparency. I'll see if I can quickly fix it, and if not, I'll let you know.