allow ChartJS custom default font

[noomz]

I just added support for custom font via ENV.

[typpo]

Thank you Siriwat. I've had problems with using Chartjs globals in the past because the settings would leak and affect other chart renders. I think that may be a problem in this case. We should add a test in test/ci/charts.js to be sure.

[noomz]

Excuse me, I don't quite get what the problem is. What will it leak and how would it effect the rendering (except for the font). By the way, I will look into the test after my work hours ;)

[typpo]

Sorry, I misunderstood. I was imagining and option that allows you to set font color. That could cause problems because the Chart object persists between requests.

However, you've implemented this as an envar, which affects ALL charts anyway, so we don't have to worry about that :)