campaigns: Display a legend under burndown charts

Created by: LawnGnome

I found the burndown charts a little hard to read without hovering. This PR adds a legend to make the colour mapping more obvious. It looks like this:

On a technical level, Recharts legends use the area's stroke colour, so we also need to duplicate the fill colour into the stroke. The only effect this has on the rendered chart is that the dots that appear when you mouseover are now in the area colour, which is arguably better anyway.