MAINT: Use color_labels; scale by 255

[jvkersch]

The "download link" in app2 (manually correcting segmentation) produces an image that is very dark; see screenshot. I believe this is because the call to array_to_data_url uses new_labels but should be using color_labels. This PR changes that, and scales color_labels to be in the range 0 to 255 (needed for PIL.Image.fromarray). With these changes, a clearer (yet somewhat garish) image of the labels is produced.

Before

After