Possibility of image selection for preview in datasets

[FabianMauz]

It should be possible for users to choose which image should be used for the preview thumbnails in analyses.

E.g.

Analysis has 2 datasets

dataset-1
-- Cat-1.jpg
-- Cat-2.jpg
dataset-2
-- Cat-3.jpg

The current behavior is taken the last image from the first dataset.
Maybe the user can also set if he likes to see the original or the annotated image if one exists.

This issue is a sequel issue from #1900 .