add keyboard shortcut for #819

[jewel-andraia]

following up #819, keyboard shortcuts shift-[up/down/left/right] for moving the (first visible) image in current selection around

[gablank]

Working on this, but I'm not sure if I understand what you mean by first visible? Can there be more than one image showing for a particular submission?

[jewel-andraia]

Comments can have multiple images.

[gablank]

Yeah, I kind of figured that out, in a way.

So, I decided that the image with the highest percentage of its height inside the viewport is the one we move. It works fairly well, although I'm not used to navigating Reddit by keyboard so maybe someone who is should test it out.

[matheod]

Fixed by #819