Change the color of the current date period on a chart to be distinct

In the old EChart implementations, we set the current date period (day, week, month etc) to a different color (#ffd8b1) to distinguish it from the others.

This functionality is used in dashboard charts and with a plan to migrate GitLab insights away from Periscope, there is a keen preference to retain this functionality.

As per the discussion in gitlab!24661 (comment 287036380).