Pretty rendering of image list items

[rixx]

If a list of items consists of only pastes with an image/* content type, it would be nice to render them as an album of images, or at least a list of previews.

[ThomasWaldmann]

I implemented #177 (see PR #271) and a thumbnail is now visible in the list view (global list with all items as well as for list items).

For the "pretty" part of this, I likely will need some help (html, css, js) ...

We use bootstrap4 meanwhile, so a new gallery view could be implemented based on what bs4 offers.

[ThomasWaldmann]

Merged the "carousel view" for image list items. It's neither feature rich, nor pretty, PRs to improve it are welcome.