Fix and improve fill color for Issue Trends chart

With chart.js 3.x, datasets' fill attribute is false by default.

This restores the fill color, with the following change:

• "Resolved" dataset is filled from the x axis
• "Opened" dataset (= the total) is filled from the Resolved dataset (to
  avoid overlap)
• Remove fill color for the "Still open" dataset since it's the delta
  from the other 2 sets it makes more sense to only display the line.

Fixes 0034847