UX: "Show more X" should be augmented with autoloading more

[yarikoptic]

some might consider it a feature, but sparkle already requires lots of finger/mouse gymnastics. Making it easier for attendees, by simply loading more whenever "Show more X" (where X is posters in posterhall or videos in screening room) is about to be rendered, would be a nice UX improvement. (IIRC this also reflects @margulies opinion)