Overlays: color scale

Color range should have its middle as white, not as black. Alternatively, middle of the range also could be parametrized and set via admin panel.

added enhancement label

White is alreasdy default middle color. Parametrized color should be added.

changed milestone to %v12.0.0

created branch 68-overlays-color-scale

mentioned in merge request !139 (merged)

closed via commit d1a7b82b

closed via merge request !139 (merged)

mentioned in commit d1a7b82b