Request: Allow users to decide which image is the "cover" of a gallery

[Kikketer]

As I was working through the sort-by-created PR, I noticed that we pick the first image with the highest sort_order. However the sort_order of a specific image is not possible to set!

So this basically equates to having the gallery cover image be the first image that's uploaded in a gallery.
https://github.com/humhub-contrib/gallery/blob/master/models/CustomGallery.php#L45

This proposal is to make a dropdown item available on an image and add a "Make Gallery Cover" option. This would then designate that image as the cover image. sort_order would be removed as it's not used today for individual images.

[AlbiWe]

double of #4