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]
I need an easy button. :smile: 
https://github.com/concord-consortium/lab-grapher/pull/10
More small commits are coming to lab and lab-interactive site to update them.