Allow double-sided scans to be flipped

[fspoettel]

We have scans of both sides for double-sided cards like investigators or acts. It would be nice if these could be flipped via an icon button in the bottom right corner of the scan when diplayed in scan mode.

The attributes back_link_id or double_sided can be used to infer the url of the backside.

• Allow double-sided scans to be flipped.
• Only allow flipping when rendered in a card list; do not allow flipping double sided cards in modals or on card pages.

animation-wise, something like this would be nice https://codepen.io/jonnowitts/pen/YdYjmd.